Each year we review the records for every working Social Security beneficiary to see if the additional earnings will increase their monthly benefit amounts. If an increase is due, we calculate your new benefit amount and pay the increase retroactive to January following the year of earnings. For example, if you have earnings in 2020 that will increase your monthly benefit amount, we will increase your benefit amount retroactive to January 2021 once we review your record.
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https://faq.ssa.gov/en-us/Topic/article/KA-01963