Form - Field Formula - Set a date to NULL

Form - Field Formula - Set a date to NULL


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

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