BPC Dates

Hello Folks

I'm starting developing some HR Input Schedulles.

I've got a question:

Imagine I have

Employee START_DATE and END_DATE

JohnBarnes, 01.01.2015, 03.01.2015

MarkHughs, 01.01.2015, 31.12.9999

How can I store this information in BPC?

One of reasons I want to do is

if END_DATE - START Date is < 90 days, Active Value = 1

else Active Value is 2

Is there any guide for this? because I can store only in numbers and not in DD/mm/YYYY wise

Thanks.

Regards,