Hi - I need your expert help. This should be easy-but cannot figure it out

I have a simple spreadsheet that keeps track of "items in", "items out" and
"carry over" by day. Since I do it by month, the formula shows the "carry
over" number all through the end of the month. I do not want it to show up
until the date is current, and then not go away on the previous columns.
ie.
Date 4/1/10 4/2/10 4/3/10 4/4/10 4/5/10 4/6/10 4/7/10
Work in 10 10 12
Work out 8 5 18
Carry Over 2 7 1 1 1 1 1

Any help would be greatly appreciated! ct