[OFF: logical question] rounding in steps of 15

Excuse me, I know my question is not PHP specific, but I hope there are some logical and mathematical talents in here, since I've always been fighting with maths. ;-) I have to provide a textfield, where workers enter their time they worked on a certain subject. The smallest degree should be 15 minutes, but the workers may enter every time, such as 357 or maybe 1:38. Is there any better way to determine the next 15min step, than to take for example 38 % 15 = 8 and see whether the result is nearer to 0 or to 15? In this case (8) 8 + 8 would be 16 so i would save 45 minutes of work. These are just informations for calculation of upcoming projects, so I don't have to check any legal rules for rounding work time or something like this. I think my way is quite complicated, so I hope anyone of you knows a better possibility to round to a certain degree. Thanks in advance, Norbert