Is it Ritual (Original title: Shiki-Jitsu) (2000)?
From Wikipedia:
The film follows a young Director returning to his home city of Ube in Yamaguchi Prefecture, and an eccentric young Woman he meets, whose quirks include saying “tomorrow is my birthday” every day and wearing very unusual clothing.
But as the days go by, it appears that the Woman has little touch with reality and is constantly escaping into a fantasy world, while the Director himself is a former anime director who is seeking to do a “real film” and embrace reality. The two eventually fall in love.
In the end, the Director confronts the Woman with her mother, allowing the Woman to make the first steps into the real world. The films ends with the Girl circling December 7 as her real birthday and the words “beyond the 33rd day: unknown”. (https://en.wikipedia.org/wiki/Shiki-Jitsu)