UTC time, GMT time, daylight saving time

It is often confused with this, so I specially studied it and recorded it here as a memo.
The entire earth is divided into twenty-four time zones, each with its own local time. In international radio communication occasions, for the sake of unification, a unified time is used called Universal Time Coordinated (UTC, Universal Time Coordinated). UTC is the same as Greenwich Mean Time (GMT, Greenwich Mean Time), which is the same as the local time in London, England. In this article, UTC means exactly the same as GMT.

The Beijing time zone is the East Eighth zone, which is eight hours ahead of UTC, and is recorded as +0800 in the Date field of the email header. If there is this line in the header of the email:
Date: Fri, 08 Nov 2002 09:42:22 +0800

The local time indicating where the letter was sent is on Friday, November 8, 2002, at 9:42 a.m. (22 seconds), and the local time of this place is eight hours ahead of UTC (+0800, is the East Eight District time). The Date field in the email header uses a twenty-four hour clock instead of AM and PM to mark morning and afternoon.
Take the sending time of this email as an example. If you want to convert this time to UTC, you can use the following formula:
UTC + time zone difference = local time

The time zone difference is positive in the east and negative in the west. Here, record the time zone difference of East Eighth District as +0800,

UTC + (+0800) = local (Beijing) time(1)
Then, UTC = local time (Beijing time) - 0800 (2)
0942 - 0800 = 0142
That is, UTC is one forty-two minutes and twenty-two seconds in the morning of that day. If the result is negative, it means the day before UTC, adding 2400 to the negative number is the time in UTC on the previous day.

For example, local (Beijing) time is 0432 (4:32 am), then UTC is 0432 - 0800 = -0368, the minus sign means the previous day, -0368 + 2400 = 2032, which is the night of the previous day Eight thirty-two.

 
New York's time zone is West 5th, which is five hours behind UTC and is recorded as -0500:
UTC + (-0500) = New York time (3)
UTC = New York time + 0500 (4)
Comparing equations (2) to (4),
UTC = Beijing time - 0800 = New York time + 0500 (5)
That is, Beijing time = New York time + 1300 (6)

That is, Beijing time is thirteen hours ahead of New York time. By formula (6),
New York time = Beijing time - 1300 (7)

summer time
In late April, New York switched to Daylight Saving Time, also known as Daylight Saving Time, an hour ahead of standard New York time, which actually became the standard time for the West Fourth, which became -0400.
UTC + (-0400) = New York Daylight Saving Time, applying the above formula,
Beijing Time = New York Daylight Saving Time + 1200
New York Daylight Saving Time = Beijing Time - 1200
Among these transformations, the most important formula is
UTC + time zone difference = local time
The time zone difference is positive in the east and negative in the west.

For example, East Eighth District (Beijing) is +0800, West Fifth District (New York) is -0500, California is West Eighth District, -0800, Central US Time Zone is West Sixth District, -0600, US Mountain Time Zone is West Seventh District , -0700, the Pacific time zone is West Eighth, -0800, using daylight saving time in summer, becomes -0700. The German time zone is East 1, +0100, which becomes +0200 in summer.
Most email programs,

For example, Outlook Express, when displaying the time, the computer program converts the time into local time before displaying it. For example, the Date field of the mail is:
Date: Fri, 08 Nov 2002 09:42:22 +0800