"Today is my birthday."
I thought that birthday was a foreign tradition that Japanese adopted recently. So, I am really surprised to see they have a native word instead of some Katakana borrow.
I'm not sure, but I have heard like that. In Edo period, all Japanese had added their age at the same day. It had been the first, January.
two possibilities. first, even if it is a borrowed word, it has kanji. e.g. コーヒー/珈琲coffee. ドイツ/獨逸Germany. second, they had been made of kanji at the time.