Resin.io devices make use of the
systemd-timesyncd service to keep the system time synchronized.
When the resin.io device boots up, and before any container is run, the system will query the hardware clock to get the current time, while it will also read the timestamp value, stored in the last modification time of a special file,
/var/lib/systemd/clock. If the hardware clock is behind the value stored with
/var/lib/systemd/clock, the system will forcefully set the clock to the stored value. This is done to ensure that time from the point of view of the applications is monotonically increasing. After that, the device will start its Network Time Protocol (NTP) client which will be attempting to sync the clock with NTP servers periodically. If a successful synchronization occurs, the last modification time of
/var/lib/systemd/clock is updated to that timestamp.
When you first provision a device, as a fallback,
/var/lib/systemd/clock is set to the timestamp of the host OS build (or more precisely, the timestamp of the systemd build within the host OS). For more info, you can check the
systemd-timesyncd documentation and the timesyncd source code.
There are certain networking requirements to ensure that the NTP service can properly function and the device time may be kept synchronized.
The NTP service requires UDP port
123 to be open, see more at our network requirements page.
The NTP service connects to the following time servers by default and these need to be accessible to the device:
Interacting with the Time Service
If you want to query the current time, you can do so by using the
date utility or the datetime related functions of the standard library of your language.
If you want to learn if the system has completed at least one successful NTP synchronization since boot you can use DBUS from your container to query that information. Check the "Checking if device time is NTP synchronized" example. The
NTPSynchronized property will tell you if you're running with a potentially stale clock or if the system is synced.