Adding a Grace Period to Fixed Expiration Date

I’m using the code at https://github.com/strangerstudios/pmpro-set-expiration-dates to set the expiry date for memberships, let’s say it’s 31 December each year and I have memberships for 1, 2 or 3 years. I’d like to be able to have a grace period of say 2 months so that if a user signs up towards the end of the membership period the expiration date is extended by one year. So using the above examples if a user signed up for a 1 year membership on 31 October 2020 it would expire on 31 December 2020, but if they signed up on 1 November 2020 it would expire on 31 December 2021. Any advice as to how to modify the code to achieve this would be very welcome, thanks.