Strange behavior in Form Rules - The option to change a work date field to the current date is unavailable.

I'm running into trouble with my form rule.  In it's current state, I have a rule that when "status" is changed to "complete" change "date complete" (a work date field) to "the current date".  This rule works fine, and displays in the "rules" box.  However, when I click to edit the "current date" choice, that very choice disappears, and I can't find a way to get it back.  In addition, i have a very similar table named "activities", and over there the option to change to current date is not available either.  Does anyone know what can be going on?