Determine when systemd service was started.
Basic operations
The easiest way to determine when service was started is to display service status.
$ systemctl status unbound
● unbound.service - Unbound DNS server Loaded: loaded (/lib/systemd/system/unbound.service; enabled; vendor preset: enabled) Active: active (running) since Mon 2021-04-05 21:11:50 CEST; 1 weeks 0 days ago Docs: man:unbound(8) Process: 359 ExecStartPre=/usr/lib/unbound/package-helper chroot_setup (code=exited, status=0/SUCCESS) Process: 380 ExecStartPre=/usr/lib/unbound/package-helper root_trust_anchor_update (code=exited, status=0/SUCCESS) Main PID: 544 (unbound) Tasks: 1 (limit: 2181) CGroup: /system.slice/unbound.service └─544 /usr/sbin/unbound -d Apr 05 21:11:50 desktop systemd[1]: Started Unbound DNS server.
You do not need to display the whole status to get this information.
$ systemctl show --property=ActiveEnterTimestamp unbound
ActiveEnterTimestamp=Mon 2021-04-05 21:11:50 CEST
$ systemctl show --property=ActiveEnterTimestamp --value unbound
Mon 2021-04-05 21:11:50 CEST
In a case like this, specify the timezone when you want to share this information.
$ TZ=GMT systemctl show --property=ActiveEnterTimestamp --value unbound
Mon 2021-04-05 19:11:50 GMT
Automation
You can calculate this value inside a shell script if you want to perform some operations on it.
Get the boot time (see how to determine when the operating system was booted).
$ awk '/btime/ {print $2}' /proc/stat
1617649812
Determine the exact moment service was started since system boot.
$ systemctl show --property=ActiveEnterTimestampMonotonic --value unbound
98510114
Calculate and display when the service was started.
$ date -d "@$(echo "1617649812 + (98510114/1000000)" | bc)"
Mon Apr 5 21:11:50 CEST 2021
Additional notes
There are different timers, so you are not limited only to the described one.
$ systemctl show unbound | grep Timestamp
WatchdogTimestampMonotonic=0 ExecMainStartTimestamp=Mon 2021-04-05 21:11:50 CEST ExecMainStartTimestampMonotonic=98502163 ExecMainExitTimestampMonotonic=0 StateChangeTimestamp=Mon 2021-04-05 21:11:50 CEST StateChangeTimestampMonotonic=98510114 InactiveExitTimestamp=Mon 2021-04-05 21:11:50 CEST InactiveExitTimestampMonotonic=98434450 ActiveEnterTimestamp=Mon 2021-04-05 21:11:50 CEST ActiveEnterTimestampMonotonic=98510114 ActiveExitTimestampMonotonic=0 InactiveEnterTimestampMonotonic=0 ConditionTimestamp=Mon 2021-04-05 21:11:50 CEST ConditionTimestampMonotonic=98429908 AssertTimestamp=Mon 2021-04-05 21:11:50 CEST AssertTimestampMonotonic=98431845
See org.freedesktop.systemd1
manual page.