Hi Harshil,
As suggested by Ganesh you can create customer exit variable, at back end you can write logic in CMOD to populate this Bex variable.
Variable should return low value as "first date of current month" and high value as "current date" and restrict your key figure with calendar day with you customer exit variable.
eg. 01.08.2013 - 21.08.2013
Regards
Shabnam