Formula Help

I am writing what I feel is a bit of a complicated formula where one cell points to another, and then that one is looked at by another and so on. I am stuck with one area. I have a cell that looks at the date. If the date is greater than the 15th of the month it returns a number which shows a value in yet another cell. 15 or less return a different number that relates to a different value. My issue is adding a formula to show a number if there is no date at all. Basically, I am looking to add an ISBLANK to the formula but it keeps giving me an error no matter how I do this.

Can someone please help?

Here is what I am looking at combining. 

=IF(DAY(A8)<16,15,DAY(EOMONTH(A8,0)))

=IF(ISBLANK(A8),"",A8)) In this string I would like the number value to be 15

Can I somehow combine these to show if the day is blank return a 15 and if its the 1st through the 15th it returns a 15. Anything over that returns end of month.

I hope that makes sense.