Get reliable start date for resampling
As illustrated by commit 507, the resampling needs to be able to get data in a given interval, with guaranteed availability of Tstart or last point before Tstart (so that the interpolation has the required number of points or time span). If resampling wants data between 8:30 and 10:00, with hourly measurements, it will actually get all data between 09:00 and 10:00 instead of the expected and better 08:00 -> 10:00.
This seems very hard to fix, since we can never be sure of the sampling rate (therefore, a database query is hard to formulate).