I tried a Custom Field-->Formula but the ISCHANGED() function was not an option. I assume a Workflow would work but I don't know how to set the criteria properly.
Appreciate the help!
2 answers
In the WFR you would set the evaluation criteria to "Every time the record is created or editied" then use the Formula
ISCHANGED(Num_Jobs_Posting)
and a Field update using either NOW() or TODAY() depening on whether you're using a DateTime field or a Date field.