I cannot recall having problems when setting the absolute date of a timer…
There could be a few other things that could make it look like the timer is not working - fx something later in the process which causes an incident and makes the process rollback to the timer?
Perhaps the timer is not being picked up - have you looked at this:
https://forum.camunda.io/t/timers-doesnt-work-after-server-restart/20167
BR
Michael