I have to calculate the numbers of days per month a service has been used based on a start service dated and a discharge (end) date even if the service hasn't ended yet. I have a basic formula that break downs the number of days per each month between two dates, but when the end date is blank the formula just calculates as if end date were end of year. What i need is a way to have the formula calculate that if end date is blank it calculates number of days to current day.

Here is the formula I am currently using: =MAX(0,MIN(EOMONTH(C$1,0),$B2)-MAX(C$1,$A2)+1)

Here is a sample of my data set and the results from above formula:
In Date End Date J F M A M J J A S O N D
6/1/2015 8/30/2015 0 0 0 0 0 30 31 30 0 0 0 0
6/2/2015 0 0 0 0 29 31 31 30 31 30 31 31
6/2/2015 8/28/2015 0 0 0 0 0 29 31 28 0 0 0 0
6/9/2015 0 0 0 0 22 31 31 30 31 30 31 31
6/19/2015 0 0 0 0 12 31 31 30 31 30 31 31

As you can see when no end date is available it calculates max # of days till end of year. I need it to calculate the blank days as if it were the current day. I've tried using ISBLANK but having trouble getting it to work. Please help. Thanks.