My worksheet has the following:-

In Cell B1to B100 a received invoice date is input
In Cell C1to C100 a received invoice date at cost center is input
In Cell D1to D100 the following formula is input to calculate the
number of days =DAYS360(B1,C1)

What I am trying to achieve is if the number of days
calculated is over 15 Days, staff have to issue a letter to the budget
holder to ask them to explain the lateness of the invoice.
So if I could get some sort of message box to appear this
will ensure that this is done.

I am able to run a validation and change the cell to red, however staff
still forget to issue letter.
So can anyone help me to sort this problem?

I have already tried the following but no joy, error circular ref:-


Public Function PromptPayment()
MsgBox "Please issue Prompt Payment Letter"
End Function

Then in cell D1 I input the following:-
=DAYS360(B1,C1)=IF(D115,PromptPayment(),"")

and if i just input this instead
=IF(D115,PromptPayment(),"")