How to determine when service was started

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.