I offered a way of doing that here:
An active timer, interrupted during its countdown by restarting Home Assistant, is restored when Home Assistant starts. It is restored with a new duration representing the remaining time left of its original duration minus however long Home Assistant was offline. If the timer’s duration expired while Home Assistant was offline, the timer is not restored on startup.
NOTE
Timers, and restoring them, should be used judiciously. I agree with frenck that they’re not a panacea, especially not for long intervals when better techniques are available. FWIW, I don’t use any long-duration timers.