Need help with rounding multiples of minutes in Excel

[MULTISPORT01]

Heres the situation: I am trying to create a workout spreadsheet which will show 90% of my previous best time on an exercise rounded to the nearest 15 seconds. example:

A1: 00:06:00
B2: =A1*.9

this displays 00:05:24... I would want it to display 00:05:30

I have tried a number of different equations without success... any help would be appreciated.

 

[Oeldere]

Book1
	A	B	C
1	00:06:00	0,9	00:05:30
Blad1

Cell Formulas
Range	Formula
C1	=MROUND(A1*B1,"0:00:15")