As I understand this question, there are two different date fields in two data sources and the data sources are tied together by a common field, user-id, and you would like to determine some value between the two dates. Are you trying to determine the time between dates? Here are a number of posts regarding finding differences between open/close dates - hopefully one of these is helpful: FAQ: Open & Close Dates.
If not, could you please restate the question with more details. Ideally with a workbook attached.
I hope this helps.