When a field is set to receive a system parameter of timestamp
and
a Rule is created to disable the timestamp field if it is not empty
then
the "disabled" field should not get a new timestamp when the record is updated

Currently, if a field is set to get a timestamp, there is no way to use a Rule to create a condition for this behavior: the field will always get timestamped.

I want to let people update records and timestamp them, but if they update a record twice, I want to hold on to the original timestamp instead of overwriting it.