Hi, this is my first post!

I have the following formula which works if I manually enter a date into cell F1 e.g 06/13 which displays as Jun-13 

=LOOKUP(VLOOKUP($F$1,$A$57:$B$68,2,FALSE),$I$2:$X$2,I5:X5)
However cell F1 contains =B1-90, to take the date in B1 minus 90 days to give the previous quarter. In cell B1 I have 01/09/13 which displays as Sep-13 which in turn displays Jun-13 in cell F1.

Why does my formula only work when I manually enter the date into F1? Is there a workaround to keep the =B1-90?