Conversion raises a tricky legal puzzle when it happens at the wrong time of year. Rabbi Shimon, quoted in the Mekhilta DeRabbi Yishmael (a halakhic midrash (rabbinic interpretive commentary) from approximately the 3rd century CE), tackled a specific scenario: what happens if someone converts to Judaism between the first Pesach (Passover) and the second Pesach?
The Torah provides a backup Passover — called Pesach Sheni (פסח שני), the "Second Passover" — exactly one month after the first, for anyone who was ritually impure or traveling and could not bring the Passover offering on time (Numbers 9:10-11). The question is whether a brand-new convert, who missed the first Passover because they were not yet Jewish, should bring the offering on Pesach Sheni.
Rabbi Shimon's reasoning is precise. The Torah says the convert "shall be as the citizen of the land" (Exodus 12:48). A natural-born Israelite who missed the first Passover — despite normally being obligated — brings the second Passover as a makeup. Following this logic, a convert who was not yet obligated during the first Passover should also bring the second.
The exclusion, however, is equally precise. Someone who converts between the two Pesachs falls into neither category. They were not obligated for the first (they were not yet Jewish), and by the time the second arrives, they cannot claim they "missed" the first — they simply were not part of the covenant yet.
This seemingly technical ruling reveals something profound about how the rabbis understood conversion. Becoming Jewish means entering a system of obligations that operates on a calendar. You cannot retroactively owe a debt you were not alive to incur. The convert's new life begins on the day of conversion — not a moment before.