Subtracting two counts

Ashish_Mathur thanks a lot for your solution! This is exactly what I needed!

Like in your example I made a date table from the first enrolment date until the last withdrawel date. I made two connections to the fact table, one on the enrolment date and one on the withdrawel date. The last one being inactive. 

In my measure for the number of withdrawels I defined the relationship with USERELATIONSHIP(  'Datetable'[DATE_ID], Facttable'[DATE_ID] ). 

Then I made another measure to subtract this measure from my existing measure for the enrolments count and thats's it!

In a table or visualisation I now use the date from the new date table to show the remaining enrolments.

Greatly appreciated!