Conditional formatting if in last 15 mins

NOW() is volatile so will be recalculated any time any calculation is done. However, it will not update if the file just sits there. You will have to hit F9 to refresh it if no other calculations are happening.

This assumes that the cell has only time, not date. Can you be more specific about your data?
Applies to $A:$A

Formula:

=A1>(MOD(NOW(),1)-TIMEVALUE("00:15:00"))

If the cells have date + time

Formula:

=A1>(NOW()-TIMEVALUE("00:15:00"))