Averaging dates, doesn't see them all as numbers

From: Kevin M (kmcclement_at_NOSPAM.precisiondrilling.com)
Date: 08/20/04 

Date: Fri, 20 Aug 2004 09:23:24 -0700

Hi all, i've started to average some dates for a user. To
make sure the values to be averaged are numbers, i used:

=IF(AND(ISNUMBER(I122),ISNUMBER(A122)),I122-A122,"")

This is autofilled for about 200 rows. The issue is that
now some of the dates are not being calculated by their
serial numbers. The answer to the above formula stays
blank until i go into either I122 or A122 and reenter the
date (or just by editing and hitting enter).

Is there a way to fix this? Excel XP.

TIA,
Kevin M