[That doesn't sound like the same disorientation he just had, and Papyrus is more than content to let that go, nodding along slowly to the rest of it.]

You're already old enough, to start forgetting things. [Very helpful younger brother energy. What else could explain the sock? But, a little less obnoxiously, he suggests:] If you remember a year, but it didn't happen to your body... I guess half of you aged. You're half a year older.

[Time to pick a new birthday, Sans. Give Papyrus a little time to get some gifts, so he can reframe himself as being early instead of late.]