19 and 15 is a pretty sizable age gap, I think it deserves a tag. Sana too pure for this world as usual.
Yachiyo is 20 now, a year has passed
She is actually 21, 2 years have passed, but this is one of those "the time doesn't pass for them" type of story, so she is canonically still 19
They had two New Year parties already, time do pass