These are predefined layouts for use in Time.Format and Time.Parse. The reference time used in the layouts is the specific time:

Mon Jan 2 15:04:05 MST 2006

which is Unix time 1136239445. Since MST is GMT-0700, the reference time can be thought of as

01/02 03:04:05PM '06 -0700

To define your own format, write down what the reference time would look like formatted your way; see the values of constants like ANSIC, StampMicro or Kitchen for examples. The model is to demonstrate what the reference time looks like so that the Format and Parse methods can apply the same transformation to a general time value.

Within the format string, an underscore _ represents a space that may be replaced by a digit if the following number (a day) has two digits; for compatibility with fixed-width Unix time formats.

A decimal point followed by one or more zeros represents a fractional second, printed to the given number of decimal places. A decimal point followed by one or more nines represents a fractional second, printed to the given number of decimal places, with trailing zeros removed. When parsing (only), the input may contain a fractional second field immediately after the seconds field, even if the layout does not signify its presence. In that case a decimal point followed by a maximal series of digits is parsed as a fractional second.

Numeric time zone offsets format as follows:

-0700  ±hhmm
-07:00 ±hh:mm
-07    ±hh

Replacing the sign in the format with a Z triggers the ISO 8601 behavior of printing Z instead of an offset for the UTC zone. Thus:

Z0700  Z or ±hhmm
Z07:00 Z or ±hh:mm
Z07    Z or ±hh

The executable example for time.Format demonstrates the working of the layout string in detail and is a good reference.

Note that the RFC822, RFC850, and RFC1123 formats should be applied only to local times. Applying them to UTC times will use "UTC" as the time zone abbreviation, while strictly speaking those RFCs require the use of "GMT" in that case. In general RFC1123Z should be used instead of RFC1123 for servers that insist on that format, and RFC3339 should be preferred for new protocols. RFC822, RFC822Z, RFC1123, and RFC1123Z are useful for formatting; when used with time.Parse they do not accept all the time formats permitted by the RFCs.

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