Return Date of Next Ocurrance of Day Number

I need a formula to return the next occurrence of day of the month.

As well it needs to be at least two weeks after the original date, but can be the date that is exactly two weeks after the original date. I've been working with a forumla:

=IF(DAY(A1+14)=B1,A1+14,DATE(YEAR(A1+14),MONTH(A1+14)+1,B1))

A1=3/18/13
B1= Drop down List (1 or 15 in C1:C2)

ie. Find the next 1st of the month.
Original Date: 3/18/13
Two Weeks After: 4/1/13

Since it's also the next 1st, it returns 4/1/13

ie. Find the next 15th of the month.
Original Date: 3/18/13
Two Weeks After: 4/1/13

Should return 4/15/13, but is returning 5/15/13, because A1+14 in that case is already in the next month. I could start nesting more IF statements, but that would get messy considering this is already part of a larger formula.

Is there any way I can accomplish this cleaner without nesting more IFs?