How does the validity of multiple re-entry permits work in Thailand?

a Re-Entry permit is valid till your Admitted to Stay date and gives you the same date when you return into Thailand.

A re-entry permit (multiple or single) does not care about calendar years. It is valid from the date you get it, and keeps your current admitted to stay date alive. It does not give you any extra time than you already have on your existing visa or extension of stay. A single re-entry is invalidated as soon as you leave the country and re-enter again. A multiple re-entry is valid for the entire period your current visa or extension is valid for.

A multiple re-entry permit is valid from the time it is granted for the extent of your visa or visa extension.