Abstract: Aiming at the problem of how to precisely judge the "year" and "day" being time unit or not in scientific papers, the usage of "year" and "day" were analyzed in detail. It is pointed out that when “year” and "day" are used to be time measure unit, they should be expressed as unit symbols, i.e. a and d, respectively; when they indicate time series, counting number, habit time, and a period of time, et al, "year" and "day" are not expression of a specific time, they should be Chinese characters "year" and "day" used rather than unit symbols.

Key words: time unit