How can i ...
1) round up datetime of timetable to nearest minute (yyyy-mm-dd hh:mm) and then
2) make adjustment to datetime so that only datetime of each 15 minutes interval is shown (e.g. 2008-07-01 12:00, 2008-07-01 12:15, etc.)
3) lastly make an interpolation so that the measurements (radiation, A, B, etc.) get adjusted to the corrected 15 minutes interval datetime (adjustment from procedure (2)).