I forgot to include this from 2020 IRS publication 525, pages 18-19. For an example see page 19 "Example 19" which states ""Example 19. You retired in 2014 and receive a pension based on your years of service. On August 3, 2020, you receive a determination of service-connected disability retroactive to 2014. Generally, you could claim a refund for the taxes paid on your pension for 2017, 2018, and 2019. However, under the special limitation period, you can also file a claim for 2016 as long as you file the claim by August 3, 2021. You can't file a claim for 2014 and 2015 because those tax years began more than 5 years before the determination.""""
