Hi,
This is not as simple as it seems (and SCN makes it harder because I cannot paste into this editor......).
You need to find the start and end dates of the select options. This involves looking at the various entries and setting a start and end date variable based upon the 'included' values only. The code I have attached does not handle patterns but you should also check if the option is 'CP' and use a suitable date for that.
I'm assuming that you know how to add columns to an ALV grid so I won't describe that bit.
A simpler way would be to try and find a table that contains ALL dates...... 
Regards
Rich