After converting a systemd timer from OnCalendar to OnBootSec timing timer unit may report a status of "elapsed" and not trigger the configured event.

Issue

An existing systemd timer unit was modified in-place, changing the OnCalendar timing to OnBootSec.

After doing so the timer consistently reports a status of "active(elapsed)" and does not trigger the event the timer is configured to manipulate.

This is only known to happen if both of the following are true:
1. Time of the current boot is later than original timer OnCalendar value.
2. The customized timer OnBootSec value is smaller than current uptime at time of applying the customization.

If the system is rebooted after modifying the systemd timer this state is cleared, but a reboot is not the proper resolution.