5 CFR § 839.611
What are the time limits for making an election?
November 4, 2020
(a) If your qualifying retirement coverage error was not previously corrected, you have 6 months from the date you receive notice of the error under §839.302 to make an election.
(b) If your qualifying retirement coverage error was previously corrected, the time limit for making an election expires on September 19, 2002.