The DataSet duration is defined by its' onset and offset time values (time borders).
Usually, the time ranges defined at DataSet and DataGroup level, include all sub-ordered Events.
This is important, because all DataSet based statistics only reflect data that is available within those DataSet time borders.
Note: Per default, INTERACT automatically checks and extends those time boundaries, as soon as you click Analysis - DataSet - Main Statistics 
to create a Time Line Chart. This ensures that all Events are included in the statistics.
No correction is made, if the duration covered by the DataSet exceeds that of the Events.
If you want to manually define the time borders of your DataSets, you need to change this default procedure.
Good reasons for working with fixed DataSet boundaries are:
oManually predefine DataSets that cover a specific test situation.
oAdjust the length of your DataSets, to make them exactly the same length
oExclude some of the Codes at the start or the end of a DataSet.