Let's look at the timing of the song.



She starts at sunset (heh, what they did there...);

The clock shows 12:00 (midnight, since it's dark and we have no Discord) about halfway through the video;

Her final notes are sung at dawn.



We know this is after the Fall Formal, and if it's winter it's a bad idea to be outside at night in a light jacket, a blouse and what could be jeans, or even leggings.

It's also before the beach shorts Forgotten Friendship, etc cetera, so it's not last day of school. It must be spring. Rarity would never let Sunset wear light clothing on a cold day, it would be The. Worst. Possible. Thing! Or at least in the top five...



Or I could be thinking too much.