ROS scripting has ridiculous support for dates.
You can’t add/substract them.
You need split you date on days, months, years and then do date arithmetic taking into consideration days in the months and leap years.
From one hand it is no difficult task for programmers, but admins usually not programmers %)
Not exact answer to you question but also about dates:
http://forum.mikrotik.com/t/calculate-the-difference-between-two-dates/107120/1
In you case task is easier (comparing to get difference between dates).