YY= year, MM= month.

for example, 7/15 is July 215. Look for it on the card (7/15). The validity period of the credit card is clearly marked on the face of the credit card. The validity period of the credit card is generally below the card number of the credit card. The standard format is that the month is before and the year is after. For example, 11/11 indicates that the validity period of the credit card is November 211.

the year, month and day refers to the time unit. People formed the concept of "day" according to the phenomenon that day and night alternate due to the rotation of the earth. According to the moon's revolution around the earth, the concept of "moon" is formed, and the concept of "year" is formed according to the alternation of seasons caused by the revolution of the earth around the sun.

on, the material movements on which these three concepts are based are independent of each other. According to accurate measurement, the time for the earth to revolve around the sun is about 365.2422 flat solar days, which is called a tropic year. And the time interval from a new moon to the next one in succession is 29.536 flat solar days, which is called a new moon.

in a year, the earth revolves around the sun once. The total length is 365 days and 1/4 days (365.2422 days), that is, 365 days, 5 hours, 48 minutes and 46 seconds, which is called "the year of the tropic".

For the convenience of application, the calendar year does not adopt tropic year, but adopts complete days. The average year of the Gregorian calendar is 365 days, and the leap year is 366 years, with a leap every four years and less leap every hundred years. The number of years used in the calendar is called "calendar year".

one year is another cycle of climate change, and the climate changes from cold to hot, and then from hot to cold; This includes the whole process of people engaged in agriculture from cultivation to harvest. Therefore, from a very early age, people used the natural law of climate change from cold to hot as the unit of calculating time and one year as the basis for making calendars. ?