As the New Year's Day is the base date in the traditional Japanese age system, care is should be taken when calculating someone's age with this method because the New Year's Day under the Gregorian calendar is different from that under the traditional Japanese calendar that had been used until the Gregorian calendar was introduced.
元日を基準とするのでグレゴリオ暦の導入以前は、暦法により元日が新暦に対し異なるため、数え年の計算には注意が必要である（単純に西暦で計算してはいけない）。