As to me this issue looks more like a support request, so I would like to mention that the issue tracker isn't the right place for that purpose. It's just for the development of Redmine. If you have any questions around Redmine or you need help, please make use of the "forums":https://www.redmine.org/projects/redmine/boards.

To answer your question...
It's not possible in vanilla Redmine, yet. You would have to write a pile of JavaScript or Ruby to achieve this.
Maybe there is some plugin out there which I doesn't know about.

Since this feature is not implemented yet I suggest to leave this issue open and change the subject to a more specific one, like "Custom date field can expire".

--------------------------------------------------------------------------------