If you're wanting to show that on the main form, you need a table field and bind the textbox to it. Your update process would update this field.
Unbound controls have no place to store data so the contents are lost when you close the object, provided that the unbound control isn't populated by an expression. Then maybe.
EDIT - I've found file attribute date/times to be unreliable as reported in Windows Explorer. That's why I would use a table field and the system date time when the operation runs.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.