Rounding Time to nearest quarter

 I was wondering if there was a way to round the time in/out to the nearest quarter hour?

This is my current formula:

var duration RoundedClockIn = Round([Clock In Date Time] - ToTimeOfDay("12:00 am"), Minutes(15));

ToTimeOfDay("12:00 am") + $RoundedClockIn

I keep receiving this error "The operator "-" can't be applied on types date/time, timeofday.

Any help would be greatly appreciated 

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Wallace Yeboah
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