From IRS article, "Is There a Deadline for Filing Form 941-X?"

"Generally, you may correct overreported taxes on a previously filed Form 941-X within 3 years of the date Form 941 was filed or 2 years from the date you paid the tax reported on Form 941, whichever is later. You may correct underreported taxes on a previously filed Form 941 if you file Form 941-X within 3 years of the date the Form 941 was filed. We call each of these time frames a 'period of limitations.' For purposes of the period of limitations, Forms 941 for a calendar year are considered filed on April 15 of the succeeding year if filed before that date.

Example. You filed your 2019 fourth quarter Form 941 on January 28, 2020, and payments were timely made. The IRS treats the return as if it were filed on April 15, 2020. On January 18, 2023, you discovered that you overreported social security and Medicare wages on that form by $350. To correct the error, you must file Form 941-X by April 18, 2023, which is the end of the period of limitations for Form 941, and use the claim process."

Q2 2020 → succeeding year 4/15/21 → Amend returns up until 4/15/24
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Q3 2020 → succeeding year 4/15/21 → Amend returns up until 4/15/24
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Q4 2020 → succeeding year 4/15/21 → Amend returns up until 4/15/24
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Q1 2021 → succeeding year 4/15/22 → Amend returns up until 4/15/25
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Q2 2021 → succeeding year 4/15/22 → Amend returns up until 4/15/25
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Q3 2021 → succeeding year 4/15/22 → Amend returns up until 4/15/25
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Q4 2021 → succeeding year 4/15/22 → Amend returns up until 4/15/25

Correcting Employment Taxes Using Form 941-X

Instructions for Form 941-X