How to subtract hours from DateTime data using DAX?

Power Pivot (and Excel) store stores dates and times as a number representing the number of days since 1900-Jan-0, plus a fractional portion of a 24 hour day: ddddd.tttttt . This is called a serial date, or serial date-time.  Value to the left of the decimal point are days.  The value to the right of the decimal point are a decimal representation of the 24 hour clock.

So in your case you can simple subtract 5 / 24 = .2083 from the date-timestamp to subtract 5 hours.

new column = table[Date-timestamp] - .2083

If you wanted to subtract 5 hours and 15 minutes from the date-timestamp you would subtract 5.25 / 24 = .2187.  If you needed to subtract 4 hours and 8 minutes, you would subtract 8 / 60 = .133 and therefore 4.133 / 24 = .17222.  And so on...

Not sure if this is the best way, but what comes to mind for me.