25 time chronyd.service | stratum | timedatectl set-time | hwclock --systohc

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In Linux there are different kinds of time
1)Hardware
2)Software
3)Network


[root@localhost tmp]# systemctl status chronyd.service 
● chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled; vendor preset: enabled)
   Active: active (running) since Fri 2017-09-22 12:53:56 EDT; 4 days ago
=====================
to write time back to hardware
[root@localhost web]# hwclock --systohc
=====================
[root@localhost tmp]# cat /etc/chrony.conf | more
# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
server 0.centos.pool.ntp.org iburst
server 1.centos.pool.ntp.org iburst
server 2.centos.pool.ntp.org iburst
server 3.centos.pool.ntp.org iburst

# Ignore stratum in source selection.
stratumweight 0
======================
Stratum 0 devices (including atomic and gps clocks) are the most accurate, but cannot be connected to via a network connection. A Stratum 0 device is usually used as a reference clock (or synchronisation source) for a Stratum 1 time server.
Stratum 1 devices are the most accurate ntp time sources accessible via a network connection. A Stratum 1 device would normally be synchronised via a Stratum 0 reference clock.
Stratum 2 devices are synchronised from a Stratum 1 device across a network connection. Due to network jitter and delays, Stratum 2 servers are not as accurate as Stratum 1 time servers. An NTP client synchronised from a Stratum 2 source would be a Stratum 3 device 
====================

[root@localhost tmp]# timedatectl
      Local time: Tue 2017-09-26 16:26:24 EDT
  Universal time: Tue 2017-09-26 20:26:24 UTC
        RTC time: Tue 2017-09-26 20:26:24
       Time zone: America/New_York (EDT, -0400)
     NTP enabled: yes
NTP synchronized: yes
 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@localhost tmp]# 



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