Manage Time service on RH and CentOS 7

When Linux system first boots the hardware clock is read first. The time on the hardware clock read from local hardware clock in UTC – Universal Time. Local time is the actual time in the current time zone. System time, unlike Hardware clock maintained by operating system. System clock is completely independent of the hardware clock.

How to manage time

Command Description
date Manage local time
hwclock Manage hardware time
timedatectl Manage time on CentOS 7 or RH 7

Examples of using date

root@on ~]# date
Wed Jun  7 13:34:40 EDT 2017
[root@on ~]# date +%d-%m-%y
07-06-17
[root@on ~]# date -s 13:40:00
Wed Jun  7 13:40:00 EDT 2017
[root@on ~]# date
Wed Jun  7 13:40:05 EDT 2017
[root@on ~]# 

Examples of using hwclock

[root@on ~]# hwclock -c
hw-time      system-time         freq-offset-ppm   tick
1496857021   1496857310.458797
1496857031   1496857320.459274                48      0
1496857041   1496857330.459571                39      0
[root@on ~]# hwclock --systohc
[root@on ~]# hwclock -c
hw-time      system-time         freq-offset-ppm   tick
1496857353   1496857353.012685
1496857363   1496857363.013179                49      0
1496857373   1496857373.013471                39      0
hwclock command meaning
hwclock -c shows the difference between hardware time and system time.
The output of this command is refreshed every 10 seconds. Listing 24.1 shows
the output of this command.
hwclock –systohc synchronizes current system time to the hardware clock.
hwclock –hctosys synchronizes current hardware time to the system clock.

Examples using timedatectl command

[root@on ~]# timedatectl
      Local time: Wed 2017-06-07 13:48:01 EDT
  Universal time: Wed 2017-06-07 17:48:01 UTC
        RTC time: Wed 2017-06-07 17:48:01
       Time zone: America/New_York (EDT, -0400)
     NTP enabled: n/a
NTP synchronized: no
 RTC in local TZ: no
      DST active: yes
 Last DST change: DST began at
                  Sun 2017-03-12 01:59:59 EST
                  Sun 2017-03-12 03:00:00 EDT
 Next DST change: DST ends (the clock jumps one hour backwards) at
                  Sun 2017-11-05 01:59:59 EDT
                  Sun 2017-11-05 01:00:00 EST
[root@on ~]# timedatectl list-timezones
Africa/Abidjan
Africa/Accra
Africa/Addis_Ababa
Africa/Algiers
Africa/Asmara
Africa/Bamako
Africa/Bangui
....
[root@on ~]# timedatectl set-timezone America/Toronto
[root@on ~]# timedatectl
      Local time: Wed 2017-06-07 13:50:09 EDT
  Universal time: Wed 2017-06-07 17:50:09 UTC
        RTC time: Wed 2017-06-07 17:50:09
       Time zone: America/Toronto (EDT, -0400)
     NTP enabled: n/a
NTP synchronized: no
 RTC in local TZ: no
      DST active: yes
 Last DST change: DST began at
                  Sun 2017-03-12 01:59:59 EST
                  Sun 2017-03-12 03:00:00 EDT
 Next DST change: DST ends (the clock jumps one hour backwards) at
                  Sun 2017-11-05 01:59:59 EDT
                  Sun 2017-11-05 01:00:00 EST
[root@on ~]# 

Stratum levels

  • To express the reliability of an NTP server, the concept of stratum is used.
  • A low stratum is more reliable than a high stratum
  • On the Internet, many stratum 1 servers that are publicly accessible are available.
  • Stratum 10 is commonly assigned to the internal clock.
  • If NTP synchronization could not be established on a server, this server typically will show a stratum 16