How to use real life timer

What i am trying to do: event in game like a boss spawn, it got killed. the time when it is killed is recorded in real life time. After 24 hours or 6 hours of real life time, it will re-spawn once a player enter the map.

I had read this:

I can records the time when boss get killed. do not know how to check if 24 hours or 6 hours has passed after boss gotten kill.

prefer help with the codes in java, or C#

any help would be much appreciated.

If you already know how to get the current time with datetime.now, just do the same thing in an update event, and compare the death tiem with the current time, and respawn if the desired time has elapsed.

-Larry