This has always been a sticking point for me.

"Section 6511(a) requires a taxpayer to file a claim for credit or refund within three years from the time the taxpayer’s return was filed or two years from the time the tax was paid, whichever period expires later."

"The time the tax was paid" means 12/31/xx for a non-filed return, i.e., the end of the calendar year for a W2 employee. It also means if you filed timely and paid tax due, and you later amended to get back that money (due to missing or mistaken info), you have two years from THAT payment date.

Even though it is "whichever period expires later," what does that mean when a return was never filed? 

"Three years from the time the taxpayer’s return was filed" CANNOT mean that if you file a 2018 return (due in 2019) in 2024 that you have three years from 2024 to file a claim. I believe that this three year period means 4/15/19 (plus extension) plus three years from then, or 2022. If you file a first return in 2023, you are too late.

If you didn't file the 2018 return until 2021 to get the W2 withholdings, you cannot get a refund. It is past two years from the time the tax was paid.