In such case, you need an unique identifier which may be employee code or a primary key of that table, you would like to insert Salary End date but only to that employee of which you are entering a new record, correct? So this should become a criteria and this criteria will be used in a update query.
So, when you press a button, first an update query will run which will search employeeID of active form in table, insert todays date. Then you would ask to go for new record.
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