There is a famous שיטת Rashi that the Matzas mitzva needs to be baked erev Pesach after חצות, otherwise you are not יוצא. Rashi gets this from the Gemara on Pesachim 120b where Rava has a היקש between מצה and קרבן פסח, that the זמן of eating מצה is the same as the זמן of eating קרבן פסח. Rashi understands that the היקש would likewise apply to the preparations, just like the קרבן פסח has to be prepared on erev Pesach after חצות, so too does the מצה. This is quoted as a יש אומרים in Shulchan Aruch. The Gra disagrees, one of his proofs is the following. What happens when erev Pesach is on Shabbos? קרבן פסח is דוחה Shabbos and is therefore brought on Shabbos afternoon, however baking מצה is not דוחה Shabbos and therefore cannot be done on Shabbos afternoon. If so, how is anyone יוצא the mitzva of מצה, the מצה was not prepared in the זמן of the קרבן פסח? In truth, those rishonim who hold like Rashi say that when erev pesach is Shabbos, you bake the מצה on the seder night. However, the Gra's question stil holds. If the מצה needs to be baked in the זמן of the קרבן פסח, the seder night is too late, you cannot bring the קרבן פסח at that time.
A number of later acharonim answer as follows. The Gemara earlier states that the קרבן פסח is brought on Shabbos (and not that night after Shabbos) because it is זמנו קבוע. Rashi explains because there is a general din that korbanos cannot be brought at night. The acharonim ask, why didn't Rashi simply answer because the Torah says that the קרבן פסח needs to be brought on the 14th of Nisan and that night is already the 15th? They answer as follows. In halacha day follows night. However, by קרבנות the din is that night follows day. Therefore they say as follows. Although the seder night is the 15th in the calendar, with regards to קרבנות it is still the 14th as night follows day. Therefore, Rashi can't answer that the seder night is too late because that night is considered the 14th for קרבנות so Rashi needs to answer that קרבנות in general cannot be brought at night.
Based on this, we can answer the Gra's question as well. From the perspective of the קרבן פסח the night of the 15th is still the זמן, there is just a technical problem that you can't bring קרבנות at night, but from the pure קרבן פסח perspective it is the זמן and therefore you can bake the מצה that night. The היקש between מצה and קרבן פסח only relates to those special dinim of קרבן פסח, not to the regular dinim of קרבנות. Therefore from that perspective the seder night is still considered to be the זמן of the קרבן פסח and therefore you can bake the מצה then.
to be continued ...