time & date

• timezone

  • timezone setup

  • tzdata installation with noninteractive

• date

  • epoch

  • timestamps

  • format

  • IOS 8601

  • rfc-3339

  • utc

  • timezone

  • common formats

• convert

  • timestamps to epoch

  • epoch to timestamps

  • convert in different timezone

  • get daylight saving

  • how many days from timestamps

  • calculate time different

  • transfer date format

  • since now

• chrony

  • install

  • conf

  • commands

  • /usr/libexec/chrony-helper

  • set local time with chrony

• systemd-timesyncd

  • install

  • config

  • commands

references:

• * imarslo: groovy time

• * iMarslo: clock/date/time

• Unix_time

timezone

timezone setup

$ sudo dpkg-reconfigure tzdata

tzdata installation with noninteractive

[!NOTE|label:references:]

• apt-get install tzdata noninteractive

# in bash
$ DEBIAN_FRONTEND=noninteractive sudo apt-get install -y tzdata

# or
$ export DEBIAN_FRONTEND=noninteractive
$ sudo apt install -y tzdata

# or
$ echo 'tzdata tzdata/Areas select Europe'       | debconf-set-selections
$ echo 'tzdata tzdata/Zones/Europe select Paris' | debconf-set-selections
$ DEBIAN_FRONTEND="noninteractive" sudo apt install -y tzdata

# or
$ sudo ln -fs /usr/share/zoneinfo/America/New_York /etc/localtime
$ export DEBIAN_FRONTEND=noninteractive
$ sudo apt-get install -y tzdata
$ sudo dpkg-reconfigure --frontend noninteractive tzdata

• or

  [!NOTE|label:references:]

  • Cingulata/Dockerfile.bfv

  $ sudo ln -snf /usr/share/zoneinfo/$(curl https://ipapi.co/timezone) /etc/localtime
  $ sudo apt install -y tzdata

• or in dockerfile

  RUN apt-get update \
      && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends tzdata

• or ENV in dockerfile

  [!NOTE|label:references:]

  • setting it via ENV should be actively discouraged

  ENV DEBIAN_FRONTEND noninteractive
  RUN apt-get update \
      && apt-get install -y --no-install-recommends tzdata

• or ARG in dockerfile

  [!NOTE|label:references:]

  • apt-get install tzdata noninteractive

  from ubuntu:bionic
  ARG DEBIAN_FRONTEND=noninteractive
  RUN apt-get update && apt-get install -y tzdata
  RUN unlink /etc/localtime
  RUN ln -s /usr/share/zoneinfo/America/New_York /etc/localtime

date

epoch

references:

• It is the number of seconds that have elapsed since the Unix epoch, minus leap seconds; the Unix epoch is 00:00:00 UTC on 1 January 1970

• What is epoch time?

$ date -u -d '1970-01-01 00:00:00' '+Normal: %F %T %:z%nUnix: %s'
Normal: 1970-01-01 00:00:00 +00:00
Unix: 0

$ date -d '1970-01-01 00:00:00' '+Normal: %F %T %:z%nUnix: %s'
Normal: 1970-01-01 00:00:00 +08:00
Unix: -28800

$ date -u -d '1970-01-01 00:00:00 UTC +1 day' '+Normal: %F %T %:z%nUnix: %s'
Normal: 1970-01-02 00:00:00 +00:00
Unix: 86400

$ date -d '2023-01-01' +%s; date -d '2023-01-01 - 1 year' +%s
1672560000
1641024000

# using now
$ date -d'now' +%s; date -d 'now - 1 day' +%s
1706952065
1706865665

$ date '+%s%3N'
1602231334983

$ date '+%s'
1602231334

timestamps

format

[!TIP]

• yyyy-MM-dd'T'HH:mm:ss.SSSZ

• yyyy-MM-dd'T'HH:mm:ss

DATE FORMAT OPTION	MEANING	EXAMPLE OUTPUT
date +%c	locale’s date time	Sat May 9 11:49:47 2020
date +%x	locale’s date	05/09/20
date +%X	locale’s time	11:49:47
date +%A	locale’s full weekday name	Saturday
date +%B	locale’s full month name	May
date +%m-%d-%Y	MM-DD-YYYY date format	05-09-2020
date +%D	MM/DD/YY date format	05/09/20
date +%F	YYYY-MM-DD date format	2020-05-09
date +%T	HH:MM:SS time format	11:44:15
date +%u	Day of Week	6
date +%U	Week of Year with Sunday as first day of week	18
date +%V	ISO Week of Year with Monday as first day of week	19
date +%j	Day of Year	130
date +%Z	Timezone	PDT
date +%m	Month of year (MM)	05
date +%d	Day of Month (DD)	09
date +%Y	Year (YY)	2020
date +%H	Hour (HH)	11
date +%H	Hour (HH) in 24-hour clock format	11
date +%I	Hour in 12-hour clock format	11
date +%p	locale’s equivalent of AM or PM	AM
date +%P	same as %p but in lower case	am

• classical date format

  $ secs=259200
  
  $ date -u -d @${secs} +"%F"
  1970-01-04
  $ date -u -d @${secs} +"%T"
  00:00:00
  
  $ date -u -d @${secs} +"%F %T"
  1970-01-04 00:00:00
  
  $ date -u -d @${secs} -Is
  1970-01-04T00:00:00+00:00

• date format with timezone

  $ date -u +"%Y-%m-%dT%H:%M:%SZ"
  2020-10-09T08:14:47Z
  
  $ date +%FT%T.%3N%:z
  2020-10-09T17:27:18.491+08:00
  
  $ date -u +"%Y-%m-%dT%H:%M:%S.%3NZ"
  2020-10-09T08:14:47.167Z
  
  $ date +%Y-%m-%d-T%H:%M:%S.%3N%z
  2020-10-09-T17:27:18.491+0800

• details

  $ date -u -d "2019-01-19T05:00:00 - 2 hours" +%Y-%m-%d_%H:%M:%S%Z --debug
  date: parsed datetime part: (Y-M-D) 2019-01-19 05:00:00 UTC-02
  date: parsed relative part: +1 hour(s)
  date: input timezone: parsed date/time string (-02)
  date: using specified time as starting value: '05:00:00'
  date: starting date/time: '(Y-M-D) 2019-01-19 05:00:00 TZ=-02'
  date: '(Y-M-D) 2019-01-19 05:00:00 TZ=-02' = 1547881200 epoch-seconds
  date: after time adjustment (+1 hours, +0 minutes, +0 seconds, +0 ns),
  date:     new time = 1547884800 epoch-seconds
  date: timezone: Universal Time
  date: final: 1547884800.000000000 (epoch-seconds)
  date: final: (Y-M-D) 2019-01-19 08:00:00 (UTC)
  date: final: (Y-M-D) 2019-01-19 08:00:00 (UTC+00)
  2019-01-19_08:00:00UTC

IOS 8601

FORMAT	EXAMPLE
YYYY	2015
YYYY-MM	2015-12
YYYY-MM-DD	2015-12-11
YYYY-MM-DD'T'hh:mmTZD	2015-12-11T20:28+01:00 or 2015-12-11T19:28Z
YYYY-MM-DD'T'hh:mm:ssTZD	2015-12-11T20:28:30+01:00 or 2015-12-11T19:28:30Z
YYYY-MM-DD'T'hh:mm:ss.sTZD	2015-12-11T20:28:30.45+01:00 or 2015-12-11T19:28:30.45Z

---

where:

• YYYY = four-digit year

• MM = two-digit month (01=January, etc.)

• DD = two-digit day of month (01 through 31)

• hh = two digits of hour (00 through 23) (am/pm NOT allowed)

• mm = two digits of minute (00 through 59)

• ss = two digits of second (00 through 59)

• s = one or more digits representing a decimal fraction of a second (i.e. milliseconds)

• TZD = time zone designator (Z or +hh:mm or -hh:mm)

$ date -I
2020-10-09

$ date -Is && date -Isecond
2020-10-09T16:31:47+08:00
2020-10-09T16:31:47+08:00

$ date -Ih
2020-10-09T16+08:00

$ date -Im
2020-10-09T16:31+08:00

rfc-3339

$ date --rfc-3339=date
2020-10-09

$ date --rfc-3339=ns
2020-10-09 17:32:14.158684000+08:00

$ date --rfc-3339=seconds
2020-10-09 17:32:14+08:00

utc

$ date
Fri Oct  9 17:09:34 CST 2020

$ date -u
Fri Oct  9 09:09:34 UTC 2020

timezone

[!NOTE|label:references:]

• list all timezone:

  $ timedatectl list-timezones | more
  
  # or
  $ tree /usr/share/zoneinfo/

• tzdb timezone descriptions

$ date '+%Z'
CST

$ date '+%z'
+0800

$ date '+%:z'
+08:00

$ date '+%::z'
+08:00:00

$ date '+%:::z'
+08

$ echo $TZ
Asia/Beijing

$ timedatectl
      Local time: Tue 2023-08-22 05:58:45 CST
  Universal time: Mon 2023-08-21 21:58:45 UTC
        RTC time: Mon 2023-08-21 21:53:46
       Time zone: Asia/Beijing (CST, +0800)
     NTP enabled: yes
NTP synchronized: yes
 RTC in local TZ: no
      DST active: n/a

$ date -d "2024-03-11" +"%Z"
PDT
$ date -d "2024-03-10" +"%Z"
PST

common formats

[!NOTE|label:references:] Shell command: date Most common Bash date commands for timestamping

FORMAT/RESULT	COMMAND	OUTPUT
YYYY-MM-DD	date -I	2020-10-09
YYYY-MM-DD_hh:mm:ss	date +%F_%T	2020-10-09_16:48:45
YYYYMMDD_hhmmss	date +%Y%m%d_%H%M%S	20201009_164845
YYYYMMDD_hhmmss (UTC version)	date --utc +%Y%m%d_%H%M%SZ	20201009_084845Z
YYYYMMDD_hhmmss (with local TZ)	date +%Y%m%d_%H%M%S%Z	20201009_164845CST
YYYYMMSShhmmss	date +%Y%m%d%H%M%S	20201009164845
YYYYMMSShhmmssnnnnnnnnn	date +%Y%m%d%H%M%S%N	20201009164845495302000
YYMMDD_hhmmss	date +%y%m%d_%H%M%S	201009_164845
Seconds since UNIX epoch:	date +%s	1602233325
Nanoseconds only:	date +%N	505337000
Nanoseconds since UNIX epoch:	date +%s%N	1602233325508581000
Nanoseconds since UNIX epoch:	date +%s%3N	1602233325508
ISO8601 UTC timestamp	date --utc +%FT%TZ	2020-10-09T08:48:45Z
ISO8601 UTC timestamp	date --utc +%FT%T%Z	2020-10-09T08:48:45UTC
ISO8601 UTC timestamp + ms	date --utc +%FT%T.%3NZ	2020-10-09T08:48:45.517Z
ISO8601 UTC timestamp + ms	date --utc +%FT%T.%3N%Z	2020-10-09T08:48:45.520UTC
ISO8601 Local TZ timestamp	date +%FT%T%Z	2020-10-09T16:48:45CST
YYYY-MM-DD (Short day)	date +%F\(%a\)	2020-10-09(Fri)
YYYY-MM-DD (Long day)	date +%F\(%A\)	2020-10-09(Friday)

convert

$ date +"%Y-%m-%dT%H:%M:%SZ"
2020-10-09T17:16:37Z

$ date -u +"%Y-%m-%dT%H:%M:%SZ"
2020-10-09T09:16:37Z

$ date -d $(date -u +"%Y-%m-%dT%H:%M:%SZ")
Fri Oct  9 17:16:37 CST 2020

HUMAN-READABLE TIME	SECONDS
1 hour	3600 seconds
1 day	86400 seconds
1 week	604800 seconds
1 month (30.44 days)	2629743 seconds
1 year (365.24 days)	31556926 seconds

timestamps to epoch

$ echo $EPOCHSECONDS
1602235097

$ date -d $(date -u +"%Y-%m-%dT%H:%M:%SZ") +%s
1602235097
$ date --date=$(date -u +"%Y-%m-%dT%H:%M:%S.%3NZ") +%s%3N
1602235097801

$ date -d '2023-01-01' +%s; date -d '2023-01-01 - 1 day' +%s
1672560000
1672473600

$ date -d '2023-01-01' +%s; date -d '2023-01-01 - 1 year' +%s
1672560000
1641024000

# using now
$ date -d'now' +%s; date -d 'now - 1 day' +%s
1706952065
1706865665

epoch to timestamps

$ date -u +"%Y-%m-%dT%H:%M:%S.%3NZ"
2020-10-09T09:18:17.795Z

$ date -d @1602235097 +%c
Fri Oct  9 17:18:17 2020

$ date -d @1602235097
Fri Oct  9 17:18:17 CST 2020

$ date -d @1602235097 -u
Fri Oct  9 09:18:17 UTC 2020

$ date -d @1602235097 -R
Fri, 09 Oct 2020 02:18:17 -0700

• via awk

  $ date -d @1602235097 +%c
  Fri Oct  9 02:18:17 2020
  
  $ echo 1602235097 | awk '{ print strftime("%c", $0); }'
  Fri Oct  9 02:18:17 2020

• epoch with 13 digits

  # wrong
  $ date -d @1718731558409 +%c
  Thu 08 Jun 56434 01:53:29 AM PDT
  
  # solution: epoch/1000
  $ date -d @$((1718731558409/1000)) +%c
  Tue 18 Jun 2024 10:25:58 AM PDT

• convert epoch with milliseconds

  Convert unix timestamp to hh:mm:ss:SSS (where SSS is milliseconds)

  d=$(date +%s%3N)
  s=${d%???}
  ms=${d#"$s"}
  date -d "@$s" +"%F %T.$ms %z"

  • result

    2020-10-09 18:28:34.534 +0800
    
    d: 1602239314534
    s: 1602239314
    ms: 534

convert in different timezone

[!NOTE|label:references:]

• CST to UTC conversion

• timezone can be found via:

  $ cat /usr/share/zoneinfo
  
  # or
  $ timedatectl list-timezones | more

$ TZ="Asia/Shanghai" date -d @$(date -d "2023-01-01 00:00:00 GMT" +"%s")
Sun Jan  1 08:00:00 CST 2023

$ TZ="America/Los_Angeles" date -d @$(date -d "2023-01-01 00:00:00 GMT" +"%s")
Sat Dec 31 16:00:00 PST 2022

• convert to another timezone

  $ date --date='TZ="GTM" 15:00 tomorrow'
  Tue Aug 22 08:00:00 PDT 2023
  
  $ echo $TZ
  America/Los_Angeles
  $ date --date='TZ="Asia/Shanghai" 16:00 tomorrow'
  Wed Aug 23 01:00:00 PDT 2023
  
  $ echo $TZ
  America/Los_Angeles
  $ TZ="Asia/Shanghai" date -d 'TZ="America/Los_Angeles" 0:00 tomorrow'
  Tue Aug 22 15:00:00 CST 2023

• convert to different timezone with daylight saving

  $ getDateTime() { echo "$1 | "$( TZ="$1" date '+%Y-%m-%d-%H-%M-%S %Z %z' ); }
  
  $ getDateTime Asia/Hong_Kong
  Asia/Hong_Kong | 2024-03-01-13-08-02 HKT +0800
  
  $ getDateTime America/Los_Angeles
  America/Los_Angeles | 2024-02-29-21-08-08 PST -0800
  
  $ getDateTime America/New_York
  America/New_York | 2024-03-01-00-08-08 EST -0500
  
  $ getDateTime Pacific/Honolulu
  Pacific/Honolulu | 2024-02-29-19-08-08 HST -1000
  
  $ getDateTime Asia/Hong_Kong
  Asia/Hong_Kong | 2024-03-01-13-08-09 HKT +0800

get daylight saving

$ date -d "2024-03-11" +%Z
PDT
$ date -d "2024-03-10" +%Z
PST

$ echo $(( ($(date -d "2024-03-10 UTC" +%s) - $(date -d "2024-03-10 PST" +%s))/(60*60) ))
-8
$ echo $(( ($(date -d "2024-03-11 UTC" +%s) - $(date -d "2024-03-11 PDT" +%s))/(60*60) ))
-7

• another

  $ if perl -e 'exit ((localtime)[8])' ; then
      echo winter
    else
      echo summer
    fi

how many days from timestamps

[!NOTE|label:references:]

• How to find the difference in days between two dates?

$ echo $(( ($(date --date="230301" +%s) - $(date --date="240301" +%s) )/(60*60*24) )) days
-366 days

# with timezone
#                PST/PDT according to daylight saving
#                                  v
$ echo $(( ($(date -d "2024-03-01 PST" +%s) - $(date -d "2023-03-01 UTC" +%s)) / (60*60*24) ))
366

calculate time different

$ date -d 'now + 3 weeks'
Fri Oct 30 20:32:04 CST 2020

$ date -d 'now + 3 weeks' +%s
1604061130

$ date -d 'Jan 1 + 11 weeks'
Wed Mar 18 00:00:00 CST 2020

$ date -d 'Jan 1 2021 + 11 weeks'
Fri Mar 19 00:00:00 CST 2021

time described by STRING

$ date -u +"%Y-%m-%dT%H:%M:%S.%3NZ" -d '90 day ago'
2020-07-11T08:14:03.145Z

$ date -u +"%Y-%m-%dT%H:%M:%S.%3NZ" -d '3 months ago'
2020-07-09T08:14:47.164Z

$ date -u -d "2019-01-19T05:00:00Z - 2 hours" +%Y-%m-%d_%H:%M:%S
2019-01-19_03:00:00

$ date -d "$(date -Iseconds -d "2018-12-10 00:00:00") - 5 hours - 20 minutes - 5 seconds"
Sun Dec  9 18:39:55 CST 2018

$ date -d "2018-12-10 00:00:00 5 hours ago 20 minutes ago 5 seconds ago"
Sun Dec  9 18:39:55 CST 2018

$ date -d '2023-01-01 - 1 year' +"%Y-%m-%d %H:%M:%S"
2022-01-01 00:00:00

# or: https://www.commandlinefu.com/commands/view/2336/get-yesterdays-date-or-a-previous-time
$ date -d '1 day ago'
$ date -d '11 hour ago'
$ date -d '2 hour ago - 3 minute'
$ date -d '16 hour'

two times different

$ seconds=$(date +%s)
$ printf "%d days %(%H hours %M minutes %S seconds)T\n" $((seconds/86400)) $seconds
18544 days 18 hours 35 minutes 48 seconds

simple one-liner

$ secs=259200
$ printf '%dh:%dm:%ds\n' $(($secs/3600)) $(($secs%3600/60)) $(($secs%60))
72h:0m:0s

• with leading zero

  $ printf '%02dh:%02dm:%02ds\n' $(($secs/3600)) $(($secs%3600/60)) $(($secs%60))
  72h:00m:00s

• with days

  $ printf '%dd:%dh:%dm:%ds\n' $(($secs/86400)) $(($secs%86400/3600)) $(($secs%3600/60))   $(($secs%60))
  3d:0h:0m:0s

• with nanoseconds

  $ printf '%02dh:%02dm:%02fs\n' $(echo -e "$secs/3600\n$secs%3600/60\n$secs%60"| bc | xargs echo)
  72h:00m:0.000000s

datediff (ddiff)

$ datediff -f "%d days, %H hours, %M mins, %S secs" "$(date +'%Y-%m-%d %H:%M:%S')" "$(date +'%Y-%m-%d %H:%M:%S' -d '3 days ago')"
-3 days, 0 hours, 0 mins, 0 secs

• or with specific format

  $ ddiff -i '%Y%m%d%H%M%S' 20190817040001 20200312000101
  17956860s
  
  $ ddiff -f "%d days, %H hours, %M mins, %S secs" -i '%Y%m%d%H%M%S' 20190817040001 20200312000101
  207 days, 20 hours, 1 mins, 0 secs

calculate with epoch

$ awk -v t=$(( $(date -d $(date +"%Y-%m-%dT%H:%M:%SZ") +%s) - $(date -d $(date +"%Y-%m-%dT%H:%M:%SZ" -d '3 days ago') +%s) )) 'BEGIN{
  printf "%d:%02d:%06.3f\n", t/3600, (t/60)%60, t%60}'
72:00:00.000

transfer date format

[!TIP]

$ date +'%Y%m%d%H%M%S'
20201009184852

$ d1=$(date +'%Y%m%d%H%M%S')
$ date --date "$(echo $d1 | sed -nr 's/(....)(..)(..)(..)(..)(..)/\1-\2-\3 \4:\5:\6/p')"
Fri Oct  9 18:48:52 CST 2020

since now

$ date -ud "1970-01-01 + 1234567890 seconds"
Fri Feb 13 23:31:30 UTC 2009

chrony

install

• from package management

  # centos/rhel
  $ sudo yum install -y chrony

• from source

  [!NOTE]

  • asciidoctor 2.0.22

  • asciidoctor is required to build docs

    $ gem install asciidoctor

  $ mkdir -p /opt/chrony
  $ git clone https://gitlab.com/chrony/chrony.git /opt/chrony && cd $_
  $ ./configure --prefix=/usr/local --mandir=/usr/share/man
  $ make && make docs
  $ sudo make install && sudo make install docs

conf

[!NOTE|label:references:]

• chrony配置

• 时钟同步

• CentOS使用Chrony部署内网NTP时间服务器

• Chrony详解：代替ntp的时间同步服务

• Systemd and ntpd problems

• ntp pool project | 授时中心

  • North America — north-america.pool.ntp.org

  • 210.72.145.44 国家授时中心

  • ntp.aliyun.com 阿里云

  • s1a.time.edu.cn 北京邮电大学

  • s1b.time.edu.cn 清华大学

  • s1c.time.edu.cn 北京大学

  • s1d.time.edu.cn 东南大学

  • s1e.time.edu.cn 清华大学

  • s2a.time.edu.cn 清华大学

  • s2b.time.edu.cn 清华大学

  • s2c.time.edu.cn 北京邮电大学

  • s2d.time.edu.cn 西南地区网络中心

  • s2e.time.edu.cn 西北地区网络中心

  • s2f.time.edu.cn 东北地区网络中心

  • s2g.time.edu.cn 华东南地区网络中心

  • s2h.time.edu.cn 四川大学网络管理中心

  • s2j.time.edu.cn 大连理工大学网络中心

  • s2k.time.edu.cn CERNET桂林主节点

  • s2m.time.edu.cn 北京大学

  • ntp.sjtu.edu.cn | 202.120.2.101 上海交通大学

• How to configure chrony as an NTP client or server in Linux

• RedHat 7: Chapter 18. Configuring NTP Using the chrony Suite

• RedHad 8: Chapter 15. Configuring time synchronization

• RedHat 9: Chapter 13. Configuring time synchronization

• How to Sync Time in CentOS 8 using Chrony

• /etc/chrony.conf

  # default
  $ cat /etc/chrony.conf | sed -r '/^(#.*)$/d' | sed -r '/^\s*$/d'
  pool 2.centos.pool.ntp.org iburst
  driftfile /var/lib/chrony/drift
  makestep 1.0 3
  rtcsync
  keyfile /etc/chrony.keys
  leapsectz right/UTC
  logdir /var/log/chrony
  
  # modified
  $ cat /etc/chrony.conf | sed -r '/^(#.*)$/d' | sed -r '/^\s*$/d'
  pool 2.rhel.pool.ntp.org iburst
  driftfile /var/lib/chrony/drift
  makestep 1.0 3
  stratumweight 0
  rtcsync
  hwtimestamp *
  allow 0.0.0.0/0
  bindcmdaddress 127.0.0.1
  bindcmdaddress ::1
  local stratum 10
  keyfile /etc/chrony.keys
  leapsectz right/UTC
  logdir /var/log/chrony
  generatecommandkey
  maxdistance 600.0

• /usr/lib/systemd/system/chronyd.service

  $ cat /usr/lib/systemd/system/chronyd.service
  [Unit]
  Description=NTP client/server
  Documentation=man:chronyd(8) man:chrony.conf(5)
  After=ntpdate.service sntp.service ntpd.service
  Conflicts=ntpd.service systemd-timesyncd.service
  ConditionCapability=CAP_SYS_TIME
  
  [Service]
  Type=forking
  PIDFile=/run/chrony/chronyd.pid
  EnvironmentFile=-/etc/sysconfig/chronyd
  ExecStart=/usr/sbin/chronyd $OPTIONS
  ExecStartPost=/usr/libexec/chrony-helper update-daemon
  PrivateTmp=yes
  ProtectHome=yes
  ProtectSystem=full
  
  [Install]
  WantedBy=multi-user.target

commands

• services

  $ sudo systemctl enable chronyd.service
  Created symlink /etc/systemd/system/multi-user.target.wants/chronyd.service → /usr/lib/systemd/system/chronyd.service.
  
  $ sudo systemctl is-active chronyd.service
  active
  
  $ sudo systemctl is-enabled chronyd.service
  enabled

• server

  $ sudo chronyd -q 'server 0.north-america.pool.ntp.org iburst'
  2024-04-02T23:51:52Z chronyd version 3.5 starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP +SCFILTER +SIGND +ASYNCDNS +SECHASH +IPV6 +DEBUG)
  2024-04-02T23:51:52Z Initial frequency -19.492 ppm
  2024-04-02T23:51:57Z System clock wrong by 0.003261 seconds (step)
  2024-04-02T23:51:57Z chronyd exiting

• chronyc

  $ chronyc sources -v
  210 Number of sources = 4
  
    .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
   / .- Source state '*' = current synced, '+' = combined , '-' = not combined,
  | /   '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
  ||                                                 .- xxxx [ yyyy ] +/- zzzz
  ||      Reachability register (octal) -.           |  xxxx = adjusted offset,
  ||      Log2(Polling interval) --.      |          |  yyyy = measured offset,
  ||                                \     |          |  zzzz = estimated error.
  ||                                 |    |           \
  MS Name/IP address         Stratum Poll Reach LastRx Last sample
  ===============================================================================
  ^- 212.227.240.160               3   8   377   103  -5334us[-5080us] +/-  101ms
  ^? ntp1.glypnod.com              2   6     3    37   -869us[ -609us] +/-   14ms
  ^* LAX.CALTICK.NET               2   7   377    36   -408us[ -147us] +/-   12ms
  ^- 131.153.171.22                2   8   377   101  -5931us[-5677us] +/-   60ms
  
  $ chronyc sourcestats -v
  210 Number of sources = 4
                               .- Number of sample points in measurement set.
                              /    .- Number of residual runs with same sign.
                             |    /    .- Length of measurement set (time).
                             |   |    /      .- Est. clock freq error (ppm).
                             |   |   |      /           .- Est. error in freq.
                             |   |   |     |           /         .- Est. offset.
                             |   |   |     |          |          |   On the -.
                             |   |   |     |          |          |   samples. \
                             |   |   |     |          |          |             |
  Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
  ==============================================================================
  212.227.240.160            25  17   31m     +0.029      0.853   -560us   586us
  ntp1.glypnod.com            2   0    64     -0.104   2000.000   -869us  4000ms
  LAX.CALTICK.NET            12   4  1099     +0.014      2.198   +654ns   614us
  131.153.171.22             25  14   31m     -0.834      2.524  -4084us  1801us
  
  $ chronyc tracking
  Reference ID    : 2D3F360D (LAX.CALTICK.NET)
  Stratum         : 3
  Ref time (UTC)  : Tue Apr 02 22:53:49 2024
  System time     : 0.000157978 seconds fast of NTP time
  Last offset     : +0.000261040 seconds
  RMS offset      : 0.001216694 seconds
  Frequency       : 19.554 ppm slow
  Residual freq   : +0.014 ppm
  Skew            : 2.387 ppm
  Root delay      : 0.024124343 seconds
  Root dispersion : 0.000968660 seconds
  Update interval : 128.2 seconds
  Leap status     : Normal
  
  $ sudo chronyc clients
  Hostname                      NTP   Drop Int IntL Last     Cmd   Drop Int  Last
  ===============================================================================
  localhost                       0      0   -   -     -      13      0   4     1

/usr/libexec/chrony-helper

$ cat /usr/libexec/chrony-helper
#!/bin/bash
# This script configures running chronyd to use NTP servers obtained from
# DHCP and _ntp._udp DNS SRV records. Files with servers from DHCP are managed
# externally (e.g. by a dhclient script). Files with servers from DNS SRV
# records are updated here using the dig utility. The script can also list
# and set static sources in the chronyd configuration file.

chronyc=/usr/bin/chronyc
chrony_conf=/etc/chrony.conf
chrony_service=chronyd.service
helper_dir=/var/run/chrony-helper
added_servers_file=$helper_dir/added_servers

network_sysconfig_file=/etc/sysconfig/network
dhclient_servers_files="/var/lib/dhclient/chrony.servers.*"
dnssrv_servers_files="$helper_dir/dnssrv@*"
dnssrv_timer_prefix=chrony-dnssrv@

. $network_sysconfig_file &> /dev/null

chrony_command() {
    $chronyc -a -n -m "$1"
}

is_running() {
    chrony_command "tracking" &> /dev/null
}

get_servers_files() {
    [ "$PEERNTP" != "no" ] && echo "$dhclient_servers_files"
    echo "$dnssrv_servers_files"
}

is_update_needed() {
    for file in $(get_servers_files) $added_servers_file; do
        [ -e "$file" ] && return 0
    done
    return 1
}

update_daemon() {
    local all_servers_with_args all_servers added_servers

    if ! is_running; then
        rm -f $added_servers_file
        return 0
    fi

    all_servers_with_args=$(cat $(get_servers_files) 2> /dev/null)

    all_servers=$(
        echo "$all_servers_with_args" |
            while read -r server serverargs; do
                echo "$server"
            done | sort -u)
    added_servers=$( (
        cat $added_servers_file 2> /dev/null
        echo "$all_servers_with_args" |
            while read -r server serverargs; do
                [ -z "$server" ] && continue
                chrony_command "add server $server $serverargs" &> /dev/null &&
                    echo "$server"
            done) | sort -u)

    comm -23 <(echo -n "$added_servers") <(echo -n "$all_servers") |
        while read -r server; do
            chrony_command "delete $server" &> /dev/null
        done

    added_servers=$(comm -12 <(echo -n "$added_servers") <(echo -n "$all_servers"))

    if [ -n "$added_servers" ]; then
        echo "$added_servers" > $added_servers_file
    else
        rm -f $added_servers_file
    fi
}

get_dnssrv_servers() {
    local name=$1 output

    if ! command -v dig &> /dev/null; then
        echo "Missing dig (DNS lookup utility)" >&2
        return 1
    fi

    output=$(dig "$name" srv +short +ndots=2 +search 2> /dev/null) || return 0

    echo "$output" | while read -r _ _ port target; do
        server=${target%.}
        [ -z "$server" ] && continue
        echo "$server port $port ${NTPSERVERARGS:-iburst}"
    done
}

check_dnssrv_name() {
    local name=$1

    if [ -z "$name" ]; then
        echo "No DNS SRV name specified" >&2
        return 1
    fi

    if [ "${name:0:9}" != _ntp._udp ]; then
        echo "DNS SRV name $name doesn't start with _ntp._udp" >&2
        return 1
    fi
}

update_dnssrv_servers() {
    local name=$1
    local srv_file=$helper_dir/dnssrv@$name servers

    check_dnssrv_name "$name" || return 1

    servers=$(get_dnssrv_servers "$name")
    if [ -n "$servers" ]; then
        echo "$servers" > "$srv_file"
    else
        rm -f "$srv_file"
    fi
}

set_dnssrv_timer() {
    local state=$1 name=$2
    local srv_file=$helper_dir/dnssrv@$name servers
    local timer

    timer=$dnssrv_timer_prefix$(systemd-escape "$name").timer || return 1

    check_dnssrv_name "$name" || return 1

    if [ "$state" = enable ]; then
        systemctl enable "$timer"
        systemctl start "$timer"
    elif [ "$state" = disable ]; then
        systemctl stop "$timer"
        systemctl disable "$timer"
        rm -f "$srv_file"
    fi
}

list_dnssrv_timers() {
    systemctl --all --full -t timer list-units | grep "^$dnssrv_timer_prefix" | \
            sed "s|^$dnssrv_timer_prefix\(.*\)\.timer.*|\1|" |
        while read -r name; do
            systemd-escape --unescape "$name"
        done
}

prepare_helper_dir() {
    mkdir -p $helper_dir
    exec 100> $helper_dir/lock
    if ! flock -w 20 100; then
        echo "Failed to lock $helper_dir" >&2
        return 1
    fi
}

is_source_line() {
    local pattern="^[ \t]*(server|pool|peer|refclock)[ \t]+[^ \t]+"
    [[ "$1" =~ $pattern ]]
}

list_static_sources() {
    while read -r line; do
        if is_source_line "$line"; then
            echo "$line"
        fi
    done < $chrony_conf
}

set_static_sources() {
    local new_config tmp_conf

    new_config=$(
        sources=$(
            while read -r line; do
                is_source_line "$line" && echo "$line"
            done)

        while read -r line; do
            if ! is_source_line "$line"; then
                echo "$line"
                continue
            fi

            tmp_sources=$(
                local removed=0

                echo "$sources" | while read -r line2; do
                    if [ "$removed" -ne 0 ] || [ "$line" != "$line2" ]; then
                        echo "$line2"
                    else
                        removed=1
                    fi
                done)

            [ "$sources" == "$tmp_sources" ] && continue
            sources=$tmp_sources
            echo "$line"
        done < $chrony_conf

        echo "$sources"
    )

    tmp_conf=${chrony_conf}.tmp

    cp -a $chrony_conf $tmp_conf &&
        echo "$new_config" > $tmp_conf &&
        mv $tmp_conf $chrony_conf || return 1

    systemctl try-restart $chrony_service
}

print_help() {
    echo "Usage: $0 COMMAND"
    echo
    echo "Commands:"
    echo "  update-daemon"
    echo "  update-dnssrv-servers NAME"
    echo "  enable-dnssrv NAME"
    echo "  disable-dnssrv NAME"
    echo "  list-dnssrv"
    echo "  list-static-sources"
    echo "  set-static-sources < sources.list"
    echo "  is-running"
    echo "  command CHRONYC-COMMAND"
}

case "$1" in
    update-daemon|add-dhclient-servers|remove-dhclient-servers)
        is_update_needed || exit 0
        prepare_helper_dir && update_daemon
        ;;
    update-dnssrv-servers)
        prepare_helper_dir && update_dnssrv_servers "$2" && update_daemon
        ;;
    enable-dnssrv)
        set_dnssrv_timer enable "$2"
        ;;
    disable-dnssrv)
        set_dnssrv_timer disable "$2" && prepare_helper_dir && update_daemon
        ;;
    list-dnssrv)
        list_dnssrv_timers
        ;;
    list-static-sources)
        list_static_sources
        ;;
    set-static-sources)
        set_static_sources
        ;;
    is-running)
        is_running
        ;;
    command|forced-command)
        chrony_command "$2"
        ;;
    *)
        print_help
        exit 2
esac

exit $?

set local time with chrony

[!NOTE|label:references:]

• chrony 详解

$ sudo timedatectl set-time "2020-02-23 12:23:01"                # <<========设置系统时间，因为开启了时间同步所以报错
Failed to set time: Automatic time synchronization is enabled
$ sudo systemctl stop chronyd
$ sudo timedatectl set-time "2020-02-23 12:23:01"                # <<==========stop chronyd 后修改系统时间，报错依旧
Failed to set time: Automatic time synchronization is enabled
$ sudo systemctl status chronyd
● chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled; vendor preset: enabled)
   Active: inactive (dead) since 四 2021-04-15 15:45:37 CST; 21s ago
     Docs: man:chronyd(8)
           man:chrony.conf(5)
  Process: 13722 ExecStartPost=/usr/libexec/chrony-helper update-daemon (code=exited, status=0/SUCCESS)
  Process: 13716 ExecStart=/usr/sbin/chronyd $OPTIONS (code=exited, status=0/SUCCESS)
 Main PID: 13720 (code=exited, status=0/SUCCESS)
 ...

$ date
2021年 04月 15日 星期四 15:46:13 CST

$ sudo timedatectl set-ntp false
$ sudo timedatectl set-time "2020-02-23 12:23:01"
$ sudo systemctl status chronyd
● chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; disabled; vendor preset: enabled)
   Active: inactive (dead)
     Docs: man:chronyd(8)
           man:chrony.conf(5)
           ...

$ sudo timedatectl status      　　　　                      # <<============ 显示当前系统和RTC设置
      Local time: 日 2020-02-23 12:23:39 CST
  Universal time: 日 2020-02-23 04:23:39 UTC
        RTC time: 日 2020-02-23 04:23:39
       Time zone: Asia/Shanghai (CST, +0800)
     NTP enabled: no
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a
$ sudo timedatectl set-ntp true
$ sudo timedatectl status
      Local time: 日 2020-02-23 12:24:14 CST
  Universal time: 日 2020-02-23 04:24:14 UTC
        RTC time: 日 2020-02-23 04:24:14
       Time zone: Asia/Shanghai (CST, +0800)
     NTP enabled: yes
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a
$ sudo systemctl start chronyd

$ sudo timedatectl status
      Local time: 四 2021-04-15 15:48:52 CST
  Universal time: 四 2021-04-15 07:48:52 UTC
        RTC time: 日 2020-02-23 04:24:44
       Time zone: Asia/Shanghai (CST, +0800)
     NTP enabled: yes
NTP synchronized: yes
 RTC in local TZ: no
      DST active: n/a
$ sudo timedatectl set-time "2020-02-23 12:23:01"
Failed to set time: Automatic time synchronization is enabled

$ sudo timedatectl set-ntp false      　　　　               # <<============= 禁用基于NTP的网络时间同步
$ sudo systemctl status chronyd
● chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; disabled; vendor preset: enabled)
   Active: inactive (dead)
     Docs: man:chronyd(8)
           man:chrony.conf(5)
           ...

$ sudo timedatectl set-time "2020-02-23 12:23:01"　　　　　　# <<=========== 再次设置时间成功
$ sudo timedatectl set-ntp true  　　　　                    # <<============ 启用基于NTP的网络时间同步

$ sudo systemctl status chronyd
● chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled; vendor preset: enabled)
   Active: active (running) since 日 2020-02-23 12:23:25 CST; 4s ago
     Docs: man:chronyd(8)
           man:chrony.conf(5)
  Process: 16110 ExecStartPost=/usr/libexec/chrony-helper update-daemon (code=exited, status=0/SUCCESS)
  Process: 16103 ExecStart=/usr/sbin/chronyd $OPTIONS (code=exited, status=0/SUCCESS)
 Main PID: 16105 (chronyd)
    Tasks: 1
   CGroup: /system.slice/chronyd.service
           └─16105 /usr/sbin/chronyd
           ...

$ date
2021年 04月 15日 星期四 15:52:14 CST

systemd-timesyncd

[!NOTE]

• issue in chrony with timedatactl

  $ timedatectl
                 Local time: Tue 2024-04-02 16:11:02 PDT
             Universal time: Tue 2024-04-02 23:11:02 UTC
                   RTC time: Tue 2024-04-02 23:11:02
                  Time zone: America/Los_Angeles (PDT, -0700)
  System clock synchronized: yes
                NTP service: n/a
            RTC in local TZ: no
  
  $ sudo timedatectl set-ntp on
  Failed to set ntp: NTP not supported

• systemd-timesyncd

install

[!NOTE|label:references:]

• * iMarslo: epel install

# centos 8
$ sudo dnf reinstall https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
$ sudo yum install -y systemd-timesyncd

• enable and start services

  $ sudo systemctl enable systemd-timesyncd.service
  Created symlink /etc/systemd/system/dbus-org.freedesktop.timesync1.service → /usr/lib/systemd/system/systemd-timesyncd.service.
  Created symlink /etc/systemd/system/sysinit.target.wants/systemd-timesyncd.service → /usr/lib/systemd/system/systemd-timesyncd.service.
  
  $ sudo systemctl start systemd-timesyncd.service
  
  $ sudo systemctl is-active systemd-timesyncd.service
  active
  $ sudo systemctl is-enabled systemd-timesyncd.service
  enabled

config

[!NOTE]

• NTP Pool Project

• timesyncd.conf

• systemd-timesyncd.service

$ cat /etc/systemd/timesyncd.conf | sed -r '/^(#.*)$/d' | sed -r '/^\s*$/d'
[Time]
NTP=0.north-america.pool.ntp.org 1.north-america.pool.ntp.org 2.north-america.pool.ntp.org 3.north-america.pool.ntp.org
FallbackNTP=0.pool.ntp.org 1.pool.ntp.org 0.fr.pool.ntp.org

# or
$ sudo systemd-analyze cat-config systemd/timesyncd.conf | sed -r '/^(#.*)$/d' | sed -r '/^\s*$/d'
[Time]
NTP=0.north-america.pool.ntp.org 1.north-america.pool.ntp.org 2.north-america.pool.ntp.org 3.north-america.pool.ntp.org
FallbackNTP=0.pool.ntp.org 1.pool.ntp.org 0.fr.pool.ntp.org

commands

• show-timesync

  $ timedatectl show-timesync --all
  LinkNTPServers=
  SystemNTPServers=0.north-america.pool.ntp.org 1.north-america.pool.ntp.org 2.north-america.pool.ntp.org 3.north-america.pool.ntp.org
  RuntimeNTPServers=
  FallbackNTPServers=0.pool.ntp.org 1.pool.ntp.org 0.fr.pool.ntp.org
  ServerName=0.north-america.pool.ntp.org
  ServerAddress=104.131.155.175
  RootDistanceMaxUSec=5s
  PollIntervalMinUSec=32s
  PollIntervalMaxUSec=34min 8s
  PollIntervalUSec=1min 4s
  NTPMessage={ Leap=0, Version=4, Mode=4, Stratum=2, Precision=-30, RootDelay=3.097ms, RootDispersion=6.774ms, Reference=408E7A25, OriginateTimestamp=Tue 2024-04-02 16:59:33 PDT, ReceiveTimestamp=Tue 2024-04-02 16:59:33 PDT, TransmitTimestamp=Tue 2024-04-02 16:59:33 PDT, DestinationTimestamp=Tue 2024-04-02 16:59:33 PDT, Ignored=no PacketCount=1, Jitter=0 }
  Frequency=1375360

• timesync-status

  $ timedatectl timesync-status
         Server: 104.131.155.175 (0.north-america.pool.ntp.org)
  Poll interval: 1min 4s (min: 32s; max 34min 8s)
           Leap: normal
        Version: 4
        Stratum: 2
      Reference: 408E7A25
      Precision: 0 (-30)
  Root distance: 8.322ms (max: 5s)
         Offset: +4.077ms
          Delay: 24.042ms
         Jitter: 0
   Packet count: 1
      Frequency: +20.986ppm

• check log

  $ journalctl -u systemd-timesyncd --no-hostname --since "1 day ago"