Thank you Khawar. I didn't know that one and it is very interesting. However I don't see how I could make use of it in my case, because I don't have a crosstable, and I cannot turn my data into a crosstable.
Indeed I can't predefine the number of month between the start date and the end date, so I wouldn't know how many columns of months I would need in my crosstable.
I'm looking at intervalmatch though, it looks like something I could use!