create a field into which an auto number is inserted

[Bill]

I have two fields: A Work start date, and a Resign date. I wish to create a
third field which will automatically enter a value in the record as soon as
the first two fields have a date entered.

 

[Rick Brandt]

I have two fields: A Work start date, and a Resign date. I wish to
create a third field which will automatically enter a value in the
record as soon as the first two fields have a date entered.

Can't be done at table design level. You could do so in the BeforeUpdate
event of the form used to modify records.

Note that if the desired value for the third field is "dependent" on the
other two, that is by looking at the other two you can calculate what should
be in the third then it would violate proper database practices to even have
this field in your table. Proper design would be to calculate it on the fly
as needed.