1, the 24-hour system (rule) is to divide the day from midnight to midnight into 24 hours, from the number 0 to 23(24 is the midnight that ends every day). This time recording system is the most commonly used in the world today. The format for expressing time in twenty-four hours is "hours: minutes" (for example, 0 1:23) or "hours: minutes: seconds" (0 1:23:45). Numbers less than 10 should start with zero. This zero is not necessary in the hour part, but it is also widely used. The rightmost part of "00:00:00" is the unit of seconds.

2. 24-hour system, especially in computer applications with many specific regulations. In the environment where the precision is higher than seconds, seconds can be expressed in decimal, and the part after decimal point is followed by decimal point or dot symbol, for example, 0 1:23:45.678. In 24 hours, a day starts at 00:00 midnight, and the last minute of every day starts at 23:59 and ends at 24:00. 24:00 on one day is equivalent to 00:00 on the next day. The digital clock shows 00:00 to 23:59, and never shows 24:00. In this way, the beginning of a new day can be determined very accurately, from 23:59:59.999 to 00:00:00.000. However, the representation of 24:00 can more clearly determine the end time of the day.