Question when adding years to formulas

I have a table for keeping track on contract lengths on customers and i want to get a notification when contract is up for negotiation.
So, per default the contracted time is 12,24 or 36 months. And since we have clients that signed contracts a couple of years ago, we’ve passed the original due date, but our contracts gets automatically renewed 12 months, if not terminated.

I’m struggling somewhat with organising the fields, what i want to achieve is basically:

  • Signed date
  • Expiry date (Signed date + Contract length)
  • Contract length (a # field)
  • Date for negotiation (Expiry date minus 6 months)
    But i also want that if expiry date has passed, a new expiry date 12 months in the future should be set, which also updates the ‘Date for negotiation’.
    Any ideas would be awesome!