The title pretty much says it all. How do I make a npc attack a player when the player enters a cell only if it is a certain time? I thought about forcing a dialogue and then causing it from there, but I was wondering if there is some other, smoother way.

You mean certain time of a day? Then it might be something like this:

Begin AttackAt1PMshort doOnceif ( doOnce <> 0 )    returnendifif ( GameHour < 13 )    ; 1 p.m    returnendifif ( GameHour >= 14 )  ; 2 p.m.    returnendifset doOnce to 1SetFight 100StartCombat Playerend