Timestamp when field has certain value

Timestamp when field has certain value

I would like my form to show current date and time when the field called "Master" (which is a drop-down field) has the value "Received" or "Reorder (on file)".

How do I do this? Would this be a field action within the "Master" field or a formula field? What would the code be?

Thanks for any help.

-silver