How to know if time is synced?

Question

I'm writing a software in C and it is running as a systemd service. I need the service to run after time has synced or alternatively, I need a way for my C program to identify if time has synced.

I'm not using GPS or RTC. Just the plain Raspberry (both Zero W and 3B+).

I'm using Raspbian Strecth. I was trying to use time-sync.target but it still call my service before date/time is synced.

How can I detect if system has updated its time?

Answer 1

timedatectl will show if synced

               Local time: Tue 2019-11-19 09:59:09 AEDT
           Universal time: Mon 2019-11-18 22:59:09 UTC
                 RTC time: n/a
                Time zone: Australia/Sydney (AEDT, +1100)
System clock synchronized: yes
              NTP service: active
          RTC in local TZ: no

/var/lib/systemd/timesync/clock shows last synch time (NOTE this is a zero length file - look at the timestamp)

May be /var/lib/systemd/clock on older Debian OS

NOTE Raspbian Stretch DOES NOT use ntpd

Answer 2

If you install ntpd instead of timesyncd, you can check out the first line of the output of ntpstat:

$ ntpstat | head -n1
unsynchronised

... later on ..

$ ntpstat | head -n1
synchronised to NTP server (10.5.26.10) at stratum 2

Inside a script or a C program, use the return code:

ntpstat returns 0 if clock is synchronised. ntpstat returns 1 if clock is not synchronised. ntpstat returns 2 if clock state is indeterminant, for example if ntpd is not contactable.

Answer 3

Since systemd 239 there is a systemd-time-wait-sync.service available that you can just use in your systemd service to start After=systemd-time-wait-sync.service. For further information look at How can I delay the startup of systemd services until the datetime is set (no RTC on the Raspberry Pi). Raspbian Buster comes with systemd 241.

The problem is that you use old-stable Raspbian Stretch which comes with systemd 232 so the nice time wait service isn't available in that version. For this situation you can look at the journal for a message about time syncing. I have made a service for this so you can also use it to start After= it. You will find it also at How can I delay the startup of systemd services until the datetime is set (no RTC on the Raspberry Pi).