NTP and hwclock

Preface

Most of Servers and PC's that are today available have built in HW clock (but not all, Raspberry PI doesn’t have it). This type of time source is realized with HW that need permanent power connect (usually a battery). According to this theoretically it shall provide us reference time source. On another hand I need to say that most of this HW clock are not providing really accurate time. According to this it is possible to manually correct local HW time.

hwclock

This application is able directly (with help of BIOS) communicate with your HW Clock.

–show

Read and present the current configuration of you HW clock

[root@NTP-Stratum-2 ~]# hwclock --show
Tue 18 Mar 2014 03:12:27 PM UTC  -0.779488 seconds

–hctosys

Read HW time and confure OS time based on HW time

[root@NTP-Stratum-2 ~]# date 110212572003                               # Change OS Time
Sun Nov  2 12:57:00 UTC 2003

[root@NTP-Stratum-2 ~]# hwclock --show                                  # Show HW time
Tue 18 Mar 2014 03:16:59 PM UTC  -0.518979 seconds

[root@NTP-Stratum-2 ~]# hwclock --hctosys                               # Configure OS time based on HW time

[root@NTP-Stratum-2 ~]# date                                            # Check the OS time
Tue Mar 18 15:17:14 UTC 2014

–systohc

Read OS time and configure HW time based on OS time.

When you are running NTP Client on this Server you can schedule it in “ cron ” to do the update of HW clock periodically

[root@NTP-Stratum-2 ~]# date 110212572003                              # Configure OS Time
Sun Nov  2 12:57:00 UTC 2003

[root@NTP-Stratum-2 ~]# hwclock --show                                 # Check HW time before change
Tue 18 Mar 2014 03:22:34 PM UTC  -0.134825 seconds

[root@NTP-Stratum-2 ~]# hwclock --systohc                              # Update HW time with OS time

[root@NTP-Stratum-2 ~]# hwclock --show                                 # Check HW time after update
Sun 02 Nov 2003 12:57:14 PM UTC  -0.722921 seconds

–compare

Compare HW and OS time

[root@NTP-Stratum-2 ~]# hwclock --compare
hw-time      system-time         freq-offset-ppm   tick
1067778037   1067778037.083645
1067778047   1067778047.084476                83      1

–utc

Keep the HW time in UTC time zone

root@NTP-Stratum-2 ~]# hwclock --utc
Tue 18 Mar 2014 03:29:47 PM UTC  -0.134741 seconds

–debug

Get the status and configuration from HW clock

[root@NTP-Stratum-2 ~]# hwclock --debug
hwclock from util-linux-ng 2.17.2
Using /dev interface to clock.
Last drift adjustment done at 1395156423 seconds after 1969
Last calibration done at 1395156423 seconds after 1969
Hardware clock is on UTC time
Assuming hardware clock is kept in UTC time.
Waiting for clock tick...
...got clock tick
Time read from Hardware Clock: 2014/03/18 15:31:35
Hw clock time : 2014/03/18 15:31:35 = 1395156695 seconds since 1969
Tue 18 Mar 2014 03:31:35 PM UTC  -0.619014 seconds
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