Undo 'Action-> Change the current user & the current date/time'

There seems no condition 'when <field> change'
the only conditions are like equal, not equal, less, after, etc.

When I create dynamic rule on the same checkbox field and timestamps fields
I could say 'When <field> in not equal to [checked]

However in the actions there is no possibilty to:
'action change <user field> [blank]'
You are forced to select a user
  But I agree for a date  this could work, I will test that scenario later (tonight)

There is a form rule that triggers "when the record is saved".  Then you get to change fields based on fields changing.  Look for that a a choice in the form rule setup for the "When" box

smile smile smile.

I woull have been surprised if you wouldn't be able to surprise me this time.
This indeed seems to open the door to what I'm looking for .

Thanks again!