Separate field ElapsedDays
(text? why do you want a number to be text? Conversion is left as an exercise for the student, if the student actually wants that. It's easy to find...)
ElapsedDays, type is calculation, Button [Options] Get(CurrentDate) - StoredDate
Button [Storage Options] Check Do not store calculation results - recalculate as needed
If you don't change the storage options, it gets run when created, and does not change. This is the key setting to get it to change when the current date changes.
If, for some bizzare reason, you really want to store the unchanging text " days" in the field as well as the number, that can also be done in the calculation, it just makes it more tedious.
GetAsText ( Get(CurrentDate) - StoredDate ) & " days"
So, 480 days later, not even an upvote? I know it's 480 days since I answered, because it works and I still have the test DB I tossed together to test it...