Expiration date of Re=entry Permit and deadline to re-enter

[sagdast 0]

Hello. My parents are green card holders and left the US prior to the issuance of their re-entry permits - which were later sent to them. It is my understanding that the re-entry permit allows you to stay out of the country for 2 years, but the actual validity of the permits exceeds that by several months.

My question is the following: Do they need to return prior to being absent for 2 years, OR prior to the expiration of the re-entry permit (which occurs several months after 2 years of their absence).

You help would be greatly appreciated