I'm not sure how much I can offer - it's a protected sheet, we are only allowed to enter some data. But there are 4 date fields and a day calculated from that.
Start Date; Release Date; First date of month, last date of month.
Here is the formula which is in column "K":
=IF(ISBLANK(H5),"",IF(ISBLANK(G5),"Enter START DATE to calculate",IF(AND(MONTH(EOMONTH($A$2,1))>MONTH(G5),MONTH(G5)>=MONTH($A$2)),IF(ISBLANK(F5,EOMONTH($A$2,0)-G5+1,H5-G5),"START DATE MUST BE >= "&TEXT($A$2,"mm/dd/yyyy"))))
Column Fields:
e= Arrival Date
f= Release Date
g=Start date
h= end date
k=#of days formula above
Based on what I've tried,.. the formula works as long as you don't enter anything in the Release date. Once you enter the release date, the #of days drops by 1.
I'm told it works 'correctly' on one computer. I've tried two computers here one with 2010 and one with 2007 and get the same result. count is off by 1 (30 days rather than 31).
I believe the MONTH variable is 30 days. That's probably where the issue is.