Before, we would floor based on the base time (midnight 1-1-1970). This
would mean that having a start date of e.g. 09:35 and a time step of
00:15 did not work well together. The start date would get rounded down
to 09:30 instead of the desired 09:35. This commit makes sure that such
cases are handled correctly, but that rounding is still performed (e.g.
09:51 will get rounded down to 09:50).