Time calculation

Hi gang,

I've already been he

http://www.mvps.org/access/datetime/date0004.htm

I have 2 time fields, "Time In" and "Time Out". I have a
third field, a number field, that is to receive a
difference of the 2. Like I said, I tried the expression
from the link above and got this error:

"The database engine does not recognize either the
field "Time In" in a validation expression, or the
default value in the table "Task" "

I'm assuming the expression from the link is to go into
the Default Value section of the intemded field. Can
anyone please explain to me,like a 4-year old, what I did
incorrectly??

Thanks very much,
Paul