In a form, how do I set a formula field date to null? 

It does not work....

If(Not(Contains(Trial_Period, 'NA')), AddDate(Dateofjoining, ToNumber(SubString(Trial_Period,1,1)), 'Month'), null)

Field - 'null' does not exist in this form

If(Not(Contains(Trial_Period, 'NA')), AddDate(Dateofjoining, ToNumber(SubString(Trial_Period,1,1)), 'Month'), 'NA')

The second and third operands of built-in function 'If()' should be of same type.

So I need to set the third operand to a date type (NULL?) 

I Tried:

If(Not(Contains(Trial_Period, 'NA')), AddDate(Dateofjoining, ToNumber(SubString(Trial_Period,1,1)), 'Month'), GetDate(NewDateTime(0,0,0,0,0,0)))

Result:

Trial Period = NA

It returns --> End of Trial Period 30-Nov-0002 (???)

Any idea? 

Thanks