If you want a start date and end date, it is a mistake to use a select option. It's a mistake often made by functional consultants as well. The reason it's a mistake is that the structure of a select option (an internal table with header line) is:
SIGN OPTION LOW HIGH
That means the user can enter all sorts of weird and wonderful combinations of selections, ranges and exclusions - and your code won't be able to handle it properly. What will you do if the user enters
I BT 01.01.2014 31.01.2014
I BT 01.01.2013 31.01.2013
Further, in your solution, you're saying that if the select option is empty, then you'll have some special logic. This is also a mistake. An empty select option in standard SAP throughout the SAP system means "select everything".
Far better (and what most SAP programs do, where they really just want a start and end date) is to use:
PARMETERS: start TYPE d, end TYPE d.