Chabad.org — English translation + Rashi
Torah Temimah (Sifrei): one might have thought a convert offers the Pesach the day he converts; the verse says “like the native” — on the fourteenth.
English: “One statute shall there be for you and for the convert and for the native of the land” — just as the native [offers] on the fourteenth, so the convert [offers] on the fourteenth.
Calendar parity for the ger — conversion does not create a private fourteenth outside the national date.
The convert is fully integrated into the same calendar and the same statute as the native-born.