VBA code to auto copy & paste special as value from target column

Hi

I have a formula that pushes a value to the current month

=IF(TEXT(TODAY(),"mmm")=A13,$C$21,0)

Unfortunately once the month changes the input reverts to 0
To resolve this I plan to do an auto copy & paste special as value to the adjoining column.
The value will go to 0 in the original cell but the copied version should stay.

To do this I need help please with some vba code
I found something similar but it is not quite correct and I am a complete novice with vba code

Any help appreciated

It is all set out fairly clearly in the attached spreadsheet

Many thanks