you could try:
From the old wiki (sorry, don't have a link here anywhere from the new documentation:
Input: year, month, day, hour, minute, second
dateTime(2007, 1, 1, 1, 1, 1)
output: "Mon Jan 01 01:01:01 CET 2007"
or, if you already have a date that you want to set the time of explicitly:
addMilliseconds(addMinutes(addHours(trimToDays($object/dateattribute), 3), 15), 50)
(which would trim it do the start of the day, then add 3 hours, then add 15 minutes and then add 50 milliseconds)