If one cell is populated refer to it, if not refer to a different cell

Stuck on this formula ( results are a date ex: 1-15-2012 ) (Got that part showing correctly :-)

AE8 = Z8 plus 90 days IF AC8 is NOT populated.

BUT

if AC8 is populated then AE8 = AC8 plus 90 days ( and ignores Z8 )

What am I doing wrong:
=INT(Z8+90)*OR(AC8+90)