Jan. is the written short form of January, the first month of the year. You use it in notes, forms, letters, and dates when you do not want to write the whole word. In American English the full stop is normal: Jan. 15. British writers sometimes write Jan without the dot, or they write January in full in careful prose. It is not a word you say out loud as “Jan-dot”; when speaking, you still say January. Always start it with a capital J, like the month.

一月（書面縮寫）