Default Value in a Form

[NotVeryTechie]

Hi there

I have a field on the form where I would like to have a default value which is the Due_Date minus 1 working day.

What I have managed so far is:
=([Due_Date]-1)

But two problems, first - this is not working days (i.e. weekdays) and secondly - the default value doesn't work. On the form I have - Nothing - in the field.

Can calculations not be used as default values?

 

[NotVeryTechie]

Just a further clarification on my post above - the Due Date is not calculated, it is given to us. So this is a value that I have and I need the value for the new field to be calculated, but there must be the option to override the calculation.