Hi Lila,
Yes, you can, using DATEDIFF, WEEKDAYDIFF, WORKMINUTESDIFF (see here for details), but since by nature such date calculations tend to get stale over time, you might also instead consider a dynamic grouping, such as this:
group.0.valueexpression=IF(DATEDIFF({actualCompletionDate},{actualStartDate})<2,"0-2",IF(DATEDIFF({actualCompletionDate},{actualStartDate})<5,"3-5",IF(DATEDIFF({actualCompletionDate},{actualStartDate})<10, "6-10","10+")))
Regards,
Doug