Code to default a date field in DOR to be a persons 18th birthday

Summary:

We have a requirement to default a document of record "Date To" field to be the 1st of the month the person turns 18. We don't to use the exact date of birth we just want to use the 1st of the month they turn 18.

The use case is for a work permit to automatically have the "Date To" default to a future date (1st of the month the person turns 18) but not have to have the HR Partner manually calculate or enter this date but see it on the page for reference and still see the document on the landing up until the 1st of the month the person turns 18 then the document can fall off the landing page view automatically and the date field does not have to be maintained manually.