Time: Read and Write 12- and 24-hour Time . 24-hour time is a system of representing time that refers to the full 24 hours in one day. Some professions use 24-hour time to reduce ambiguity between whether a time is morning or afternoon e.g. military, airline companies and medicine. . Timetables are often written using 24-hour time to avoid confusion between whether a time is in the morning or the afternoon. 24-hour time can use a colon to separate the hours and minutes (and seconds if included).

20分钟计时器，倒计时闹钟，二十分钟定时器