This is based on the fact that 15 minutes is 1/96th of a day - 24*60/15.
…to round to the nearest 15 minutes, take the time value, multiply it by 96, round it, and then divide it by 96.
For example, if the time value you wish to round is in cell E5, the following formula does the rounding very nicely:=ROUND(E5*96,0)/96
Notice that this formula uses the ROUND worksheet function, which is intrinsic to Excel and doesn’t require an add-in.
To round a number or hours to the nearest quarter hour (not a date/time number)