Math function (for use in PS script) to determine exact 5 minute intervals starting at top of the hour

This is more of a math question than PS, but I am wondering if anyone has seen or created a math function that will determine the next 5 minute time slice from starting at the top of the last hour.  Not sure how to explain what I need (which has made searching for this with Google difficult), but let me give an example:

If the time is 1:03 PM, I want a math function to spit out 1:05 PM (not 1:08 PM, which is 5 minutes from the current time)

If the time is 1:48 PM, I want that math function to spit out 1:50 PM (not 1:53 PM)

If you could provide this in PS format, that would be best - but if not, even if you could provide the math technique to figure this out, I can convert it to PS myself.

Thanks

NK