If you’re talking about game time, you can do something like this:
"Waiting"
Boring Place is a room. "You're in a boring place. Nothing cool is happening."
When play begins:
now the time of day is 12:00 PM;
now the right hand status line is "[time of day]";
the cool thing happens at 12:05 PM.
At the time when the cool thing happens:
say "A very cool thing has happened."
Check out Chapter 9 of Writing with Inform for more on this.
If you’re talking about real time, then I think you’ll have to turn to an extension (look here, check for newer versions here) or do some glulx hacking in I6. This old thread might be of interest.