Enter the 80 as hours - 80:00.

Dates and times in Excel are just formatted numbers. Dates to the left
of the decimal and times to the right.

80 hours = 3.33333333333333 (3 and 1 third days)

Excel won't display negative time/date values. (######) Make sure you
subtract the larger from the smaller if you're going to display the
result as a time/date.

Cliff Edwards