std.datetime.date.DateTimeException if the range is empty or if the generated time point is in the wrong direction (i.e. if iterating forward and the generated time point is before front, or if iterating backwards and the generated time point is after front).
Pops front from the range, using func to generate the next time point in the range. If the generated time point is beyond the edge of the range, then front is set to that edge, and the range is then empty. So, if iterating forwards, and the generated time point is greater than the interval's end, then front is set to end. If iterating backwards, and the generated time point is less than the interval's begin, then front is set to begin.