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time: rewrite the documentation for layout strings
People continue to be confused by how these work. Address that by some rejiggering. Introduce a constant called Layout that both defines the time and provides a reference point for Parse and Format to refer to. We can then delete much redundancy, especially for Format's comments, but Parse tightens a bit too. Then change the way the concept of the layout string is introduced, and provide a clearer catalog of what its elements are. Fixes #38871 Change-Id: Ib967ae70c7d5798a97b865cdda1fda4daed8a99a Reviewed-on: https://go-review.googlesource.com/c/go/+/320252 Trust: Rob Pike <r@golang.org> Run-TryBot: Rob Pike <r@golang.org> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Russ Cox <rsc@golang.org>
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Rob Pike
parent
bfe3573d58
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5e191f8f48
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@ -7,58 +7,18 @@ package time
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import "errors"
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// These are predefined layouts for use in Time.Format and time.Parse.
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// The reference time used in the layouts is the specific time:
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// Mon Jan 2 15:04:05 MST 2006
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// which is Unix time 1136239445. Since MST is GMT-0700,
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// the reference time can be thought of as
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// The reference time used in these layouts is the specific time stamp:
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// 01/02 03:04:05PM '06 -0700
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// To define your own format, write down what the reference time would look
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// like formatted your way; see the values of constants like ANSIC,
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// StampMicro or Kitchen for examples. The model is to demonstrate what the
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// reference time looks like so that the Format and Parse methods can apply
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// the same transformation to a general time value.
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// (January 2, 15:04:05, 2006, in time zone seven hours west of GMT).
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// That value is recorded as the constant named Layout, listed below. As a Unix
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// time, this is 1136239445. Since MST is GMT-0700, the reference would be
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// printed by the Unix date command as:
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// Mon Jan 2 15:04:05 MST 2006
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// It is a regrettable historic error that the date uses the American convention
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// of putting the numerical month before the day.
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//
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// Some valid layouts are invalid time values for time.Parse, due to formats
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// such as _ for space padding and Z for zone information.
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//
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// Within the format string, an underscore _ represents a space that may be
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// replaced by a digit if the following number (a day) has two digits; for
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// compatibility with fixed-width Unix time formats.
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//
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// A decimal point followed by one or more zeros represents a fractional
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// second, printed to the given number of decimal places.
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// Either a comma or decimal point followed by one or more nines represents
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// a fractional second, printed to the given number of decimal places, with
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// trailing zeros removed.
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// When parsing (only), the input may contain a fractional second
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// field immediately after the seconds field, even if the layout does not
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// signify its presence. In that case either a comma or a decimal point
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// followed by a maximal series of digits is parsed as a fractional second.
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//
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// Numeric time zone offsets format as follows:
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// -0700 ±hhmm
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// -07:00 ±hh:mm
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// -07 ±hh
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// Replacing the sign in the format with a Z triggers
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// the ISO 8601 behavior of printing Z instead of an
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// offset for the UTC zone. Thus:
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// Z0700 Z or ±hhmm
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// Z07:00 Z or ±hh:mm
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// Z07 Z or ±hh
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//
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// The recognized day of week formats are "Mon" and "Monday".
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// The recognized month formats are "Jan" and "January".
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//
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// The formats 2, _2, and 02 are unpadded, space-padded, and zero-padded
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// day of month. The formats __2 and 002 are space-padded and zero-padded
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// three-character day of year; there is no unpadded day of year format.
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//
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// Text in the format string that is not recognized as part of the reference
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// time is echoed verbatim during Format and expected to appear verbatim
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// in the input to Parse.
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//
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// The executable example for Time.Format demonstrates the working
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// of the layout string in detail and is a good reference.
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// The example for Time.Format demonstrates the working of the layout string
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// in detail and is a good reference.
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//
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// Note that the RFC822, RFC850, and RFC1123 formats should be applied
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// only to local times. Applying them to UTC times will use "UTC" as the
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@ -71,7 +31,65 @@ import "errors"
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// permitted by the RFCs and they do accept time formats not formally defined.
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// The RFC3339Nano format removes trailing zeros from the seconds field
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// and thus may not sort correctly once formatted.
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//
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// Most programs can use one of the defined constants as the layout passed to
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// Format or Parse. The rest of this comment can be ignored unless you are
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// creating a custom layout string.
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//
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// To define your own format, write down what the reference time would look like
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// formatted your way; see the values of constants like ANSIC, StampMicro or
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// Kitchen for examples. The model is to demonstrate what the reference time
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// looks like so that the Format and Parse methods can apply the same
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// transformation to a general time value.
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//
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// Here is a summary of the components of a layout string. Each element shows by
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// example the formatting of an element of the reference time. Only these values
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// are recognized. Text in the layout string that is not recognized as part of
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// the reference time is echoed verbatim during Format and expected to appear
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// verbatim in the input to Parse.
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//
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// Year: "2006" "06"
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// Month: "Jan" "January"
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// Textual day of the week: "Mon" "Monday"
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// Numeric day of the month: "2" "_2" "02"
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// Numeric day of the year: "__2" "002"
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// Hour: "15" "3" "03" (PM or AM)
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// Minute: "4" "04"
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// Second: "5" "05"
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// AM/PM mark: "PM"
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//
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// Numeric time zone offsets format as follows:
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// "-0700" ±hhmm
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// "-07:00" ±hh:mm
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// "-07" ±hh
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// Replacing the sign in the format with a Z triggers
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// the ISO 8601 behavior of printing Z instead of an
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// offset for the UTC zone. Thus:
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// "Z0700" Z or ±hhmm
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// "Z07:00" Z or ±hh:mm
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// "Z07" Z or ±hh
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//
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// Within the format string, the underscores in "_2" and "__2" represent spaces
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// that may be replaced by digits if the following number has multiple digits,
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// for compatibility with fixed-width Unix time formats. A leading zero represents
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// a zero-padded value.
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//
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// The formats and 002 are space-padded and zero-padded
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// three-character day of year; there is no unpadded day of year format.
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//
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// A decimal point followed by one or more zeros represents a fractional
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// second, printed to the given number of decimal places.
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// Either a comma or decimal point followed by one or more nines represents
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// a fractional second, printed to the given number of decimal places, with
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// trailing zeros removed.
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// For example "15:04:05,000" or "15:04:05.000" formats or parses with
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// millisecond precision.
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//
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// Some valid layouts are invalid time values for time.Parse, due to formats
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// such as _ for space padding and Z for zone information.
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//
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const (
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Layout = "01/02 03:04:05PM '06 -0700" // The reference time, in numerical order.
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ANSIC = "Mon Jan _2 15:04:05 2006"
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UnixDate = "Mon Jan _2 15:04:05 MST 2006"
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RubyDate = "Mon Jan 02 15:04:05 -0700 2006"
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@ -531,23 +549,12 @@ func (t Time) GoString() string {
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return string(buf)
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}
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// Format returns a textual representation of the time value formatted
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// according to layout, which defines the format by showing how the reference
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// time, defined to be
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// Mon Jan 2 15:04:05 -0700 MST 2006
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// would be displayed if it were the value; it serves as an example of the
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// desired output. The same display rules will then be applied to the time
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// value.
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// Format returns a textual representation of the time value formatted according
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// to the layout defined by the argument. See the documentation for the
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// constant called Layout to see how to represent the layout format.
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//
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// A fractional second is represented by adding either a comma or a
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// period and zeros to the end of the seconds section of layout string,
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// as in "15:04:05,000" or "15:04:05.000" to format a time stamp with
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// millisecond precision.
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//
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// Predefined layouts ANSIC, UnixDate, RFC3339 and others describe standard
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// and convenient representations of the reference time. For more information
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// about the formats and the definition of the reference time, see the
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// documentation for ANSIC and the other constants defined by this package.
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// The executable example for Time.Format demonstrates the working
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// of the layout string in detail and is a good reference.
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func (t Time) Format(layout string) string {
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const bufSize = 64
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var b []byte
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@ -855,21 +862,19 @@ func skip(value, prefix string) (string, error) {
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}
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// Parse parses a formatted string and returns the time value it represents.
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// The layout defines the format by showing how the reference time,
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// defined to be
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// Mon Jan 2 15:04:05 -0700 MST 2006
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// would be interpreted if it were the value; it serves as an example of
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// the input format. The same interpretation will then be made to the
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// input string.
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// See the documentation for the constant called Layout to see how to
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// represent the format. The second argument must be parseable using
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// the format string (layout) provided as the first argument.
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//
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// Predefined layouts ANSIC, UnixDate, RFC3339 and others describe standard
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// and convenient representations of the reference time. For more information
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// about the formats and the definition of the reference time, see the
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// documentation for ANSIC and the other constants defined by this package.
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// Also, the executable example for Time.Format demonstrates the working
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// of the layout string in detail and is a good reference.
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// The example for Time.Format demonstrates the working of the layout string
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// in detail and is a good reference.
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//
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// Elements omitted from the value are assumed to be zero or, when
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// When parsing (only), the input may contain a fractional second
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// field immediately after the seconds field, even if the layout does not
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// signify its presence. In that case either a comma or a decimal point
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// followed by a maximal series of digits is parsed as a fractional second.
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//
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// Elements omitted from the layout are assumed to be zero or, when
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// zero is impossible, one, so parsing "3:04pm" returns the time
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// corresponding to Jan 1, year 0, 15:04:00 UTC (note that because the year is
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// 0, this time is before the zero Time).
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@ -879,6 +884,8 @@ func skip(value, prefix string) (string, error) {
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// For layouts specifying the two-digit year 06, a value NN >= 69 will be treated
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// as 19NN and a value NN < 69 will be treated as 20NN.
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//
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// The remainder of this comment describes the handling of time zones.
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//
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// In the absence of a time zone indicator, Parse returns a time in UTC.
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//
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// When parsing a time with a zone offset like -0700, if the offset corresponds
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