Re-entry Permit - When does it end for rentry?

[Vascoda]

I am in Thailand now and I have Permitted to Stamp till 22 March, 2020.
Got a new retirement extension that is stamped Extension of Stay Permitted Upto 20 MAR 2021.
I am leaving Thailand on March 18, 2020 (before my current Permitted to Stay date).

Come back again in October for my Thailand stay (I stay only 6-month in Thailand Oct-March) using my retirement extension.
If I go to the Chonburi office and get a single re-entry permit, what will be the valid until date of the permit? (expecting it to be 20 MAR, 2021).
I know I can get one in the Airport, but want to get it in the Chonburi office in case I forget to get one at the Airport in a rush. 

[ubonjoe]

Your new re-entry permit will be valid for entry until 20 March 2021.

You new extension overrides the existing permit to stay date when it is issued.