A mismatch between Box 12 code TT and your own overtime records has three possible explanations: your expectation is wrong, the W-2 is wrong, or both are right about different things. Work through them in that order.
Step 1: check your own expectation
Code TT is the premium only, not your overtime total. Take your total overtime pay at time and a half and divide by 3. If the result is close to the code TT figure, the W-2 is probably right and the headline number was the trap. The paystub guide covers the division methods, including the divide by 4 rule for double time.
Step 2: look for the standard causes
Employers compute code TT as FLSA overtime hours times one half of the FLSA regular rate. Mismatches commonly come from: overtime your employer paid under state law or a contract beyond what the FLSA requires (not qualified, correctly excluded); a blended or weighted-average rate handled differently; overtime in the last pay period of the year being reported in the year it was paid, not earned; or a plain payroll error, which employers saw in volume during the first reporting year.
Step 3: if the W-2 is wrong, the fix is a W-2c
From 2026, the reported code TT figure is the basis of the claim (IRS FS-2026-13). If it understates your qualified premium, ask your employer, with your paystub maths in hand, to issue a corrected Form W-2c. If they will not or cannot, only the reported amount can be claimed; you cannot top it up from paystubs, and Form 4852 does not work as a substitute. If code TT overstates what you were paid, claim only the amount actually paid and ask for the correction for your records. The reconciliation tool gives the outcome for your exact pair of numbers.