In Expert Mode the end of time time intervals are defined using an Event Schedule. You can think of this end points of the time intervals as the points in time when you want to check the count of observations.

The start of each time interval is calculated by subtracting a fixed time span or a fixed number of months from the end of the interval. The time interval defined is inclusive on the lower border and exclusive on the upper. I.e. A time interval 2015-01-01:00:00 to  2015-02-01:00:00 would include observations with a sampling time stamp 2015-01-01:00:00 or  2015-01-31:23:59 but would not include observations with a sampling time stamp 2015-02-01:00:00.