first, just because it looks odd, I would convert the #100,1,1# to a date...unless your field is a string that looks like that (but then it should have quotes)
Just being cautious since it doesn't look the 'norm'
As for when the formula is evaluated, depends on where you put it. If you put in a group header/footer, it will only evaluated once per group, but if you place it in the detail section, it will evaluate for every record.
ps
you should be able to use an aggregate function on the group. I believe that there is a MAX()function which would look like:
MAX({table.field}, {group})
Edited by lockwelle - 20 Apr 2012 at 4:56am