How to compare dates from two different records

All,

I've been banging my head on this one. Here's what I'm trying to do:

I need to compare the scheduled date (phone call date) of the most recent phone call on a customer record, to a custom date field on the customer record. I am currently doing this as a customer search.

On the results tab of my search, I attempted the following:

case when to_char({call.startdate}, 'YYMMDD') > to_char({custentity_date_uploaded}, 'YYMMDD') then null else 0 end

the problem comes in with the {call.startdate} because I don't know which call the search is looking at. When I review the results, I am not seeing consistancy throughout. Some of the result set has a 0 (meaning the scheduled call is not greater/newer than the custom date field) but when I review the actual record, the customer record does have calls that have a scheduled date greater than my custom date field.