What the statute covers
The ordinary joint petition is filed during the 90 days before the second anniversary of conditional residence. Form I-751 is the petition used for covered marriage-based conditional residents. The marriage date and the date conditional residence began are different dates and should not be confused.
Limits and practical context
Section 216(c)(4) provides specified waivers of joint filing, including qualifying good-faith marriages terminated in divorce, battery or extreme cruelty, and extreme hardship. Requirements differ by basis. The joint-petition filing window should not be treated as a universal waiver filing rule. The full statute and USCIS instructions should be checked for the selected route.