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Why is there a T in the time formatting time?

If the time is: 2020-01-12T15:17:21
The International Organization for Standardization International Standardization ISO 8601 is a method of representing dates and time, and its full name is "Data storage and exchange forms·Information exchange·Date and time representation method"
The original text is as follows:

Combination representation of date and time Edit
When combining the representation, add a capital letter T in front of the time. If you want to indicate that it is 5:30:8 pm Beijing time on May 3, 2004, it can be written as 2004-05-03T17:30:08+08:00 or 20040503T173008+08.

So when the T date and time are combined, a T is added in the middle.
The Baidu address of iso 8806 is:
/item/ISO%208601/3910715?fr=aladdin