Need to add 40 to the a numeric field value after a certain number of days pasts in another date field. Can anyone please help! Thanks!

I have another field in the table where employees will enter requested vacation hours, then during the approval process the manager can subtract the hours from their available time... that much can be a manual process...

However, I've been asked for their yearly vacation renewal allowance to automatically be added to their available vacation time field.Adding to what is already there and keeping the field editable so the manager through the year can subtract hours used.  Is that possible?  Thanks