you can use the round worksheet function. the two formulae below are examples, the first rounds to the half hour and the second to the quarter.
=ROUND(A1*24/0.5 ,0)*0.5 /24
=ROUND(A1*24/0.25,0)*0.25/24
The bits in red are what you change to determine the roundup value (1/4 hour 0.25 and 1/2 hour 0.5).
If you simply want time to the nearest hour then the formula is shorter.
=ROUND(A1*24,0)/24
Don't forget the [hh]:mm formatting
hope this helps
R4