Get the start time of the next window for the current rollpoint
the start time for the next roll point an interval after the currentDateTime date
Returns the date of the next roll point after the currentDateTime date passed to the method.
The basic strategy is to subtract the time parts that are less significant than the rollpoint from the current time. This should roll the time back to the start of the time window for the current rollpoint. Then we add 1 window worth of time and get the start time of the next window for the rollpoint.