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GNU Emacs Lisp Reference Manual

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40.6 Time Conversion

These functions convert time values (lists of two or three integers) to strings or to calendrical information. There is also a function to convert calendrical information to a time value. You can get time values from the functions current-time (see section 40.5 Time of Day) and file-attributes (see section 25.6.4 Other Information about Files).

Many operating systems are limited to time values that contain 32 bits of information; these systems typically handle only the times from 1901-12-13 20:45:52 UTC through 2038-01-19 03:14:07 UTC. However, some operating systems have larger time values, and can represent times far in the past or future.

Time conversion functions always use the Gregorian calendar, even for dates before the Gregorian calendar was introduced. Year numbers count the number of years since the year 1 B.C., and do not skip zero as traditional Gregorian years do; for example, the year number -37 represents the Gregorian year 38 B.C.

Function: format-time-string format-string &optional time universal
This function converts time (or the current time, if time is omitted) to a string according to format-string. The argument format-string may contain `%'-sequences which say to substitute parts of the time. Here is a table of what the `%'-sequences mean:

`%a'
This stands for the abbreviated name of the day of week.
`%A'
This stands for the full name of the day of week.
`%b'
This stands for the abbreviated name of the month.
`%B'
This stands for the full name of the month.
`%c'
This is a synonym for `%x %X'.
`%C'
This has a locale-specific meaning. In the default locale (named C), it is equivalent to `%A, %B %e, %Y'.
`%d'
This stands for the day of month, zero-padded.
`%D'
This is a synonym for `%m/%d/%y'.
`%e'
This stands for the day of month, blank-padded.
`%h'
This is a synonym for `%b'.
`%H'
This stands for the hour (00-23).
`%I'
This stands for the hour (01-12).
`%j'
This stands for the day of the year (001-366).
`%k'
This stands for the hour (0-23), blank padded.
`%l'
This stands for the hour (1-12), blank padded.
`%m'
This stands for the month (01-12).
`%M'
This stands for the minute (00-59).
`%n'
This stands for a newline.
`%p'
This stands for `AM' or `PM', as appropriate.
`%r'
This is a synonym for `%I:%M:%S %p'.
`%R'
This is a synonym for `%H:%M'.
`%S'
This stands for the seconds (00-59).
`%t'
This stands for a tab character.
`%T'
This is a synonym for `%H:%M:%S'.
`%U'
This stands for the week of the year (01-52), assuming that weeks start on Sunday.
`%w'
This stands for the numeric day of week (0-6). Sunday is day 0.
`%W'
This stands for the week of the year (01-52), assuming that weeks start on Monday.
`%x'
This has a locale-specific meaning. In the default locale (named `C'), it is equivalent to `%D'.
`%X'
This has a locale-specific meaning. In the default locale (named `C'), it is equivalent to `%T'.
`%y'
This stands for the year without century (00-99).
`%Y'
This stands for the year with century.
`%Z'
This stands for the time zone abbreviation.

You can also specify the field width and type of padding for any of these `%'-sequences. This works as in printf: you write the field width as digits in the middle of a `%'-sequences. If you start the field width with `0', it means to pad with zeros. If you start the field width with `_', it means to pad with spaces.

For example, `%S' specifies the number of seconds since the minute; `%03S' means to pad this with zeros to 3 positions, `%_3S' to pad with spaces to 3 positions. Plain `%3S' pads with zeros, because that is how `%S' normally pads to two positions.

The characters `E' and `O' act as modifiers when used between `%' and one of the letters in the table above. `E' specifies using the current locale's "alternative" version of the date and time. In a Japanese locale, for example, %Ex might yield a date format based on the Japanese Emperors' reigns. `E' is allowed in `%Ec', `%EC', `%Ex', `%EX', `%Ey', and `%EY'.

`O' means to use the current locale's "alternative" representation of numbers, instead of the ordinary decimal digits. This is allowed with most letters, all the ones that output numbers.

If universal is non-nil, that means to describe the time as Universal Time; nil means describe it using what Emacs believes is the local time zone (see current-time-zone).

This function uses the C library function strftime to do most of the work. In order to communicate with that function, it first encodes its argument using the coding system specified by locale-coding-system (see section 33.12 Locales); after strftime returns the resulting string, format-time-string decodes the string using that same coding system.

Function: decode-time time
This function converts a time value into calendrical information. The return value is a list of nine elements, as follows:

 
(seconds minutes hour day month year dow dst zone)

Here is what the elements mean:

seconds
The number of seconds past the minute, as an integer between 0 and 59.
minutes
The number of minutes past the hour, as an integer between 0 and 59.
hour
The hour of the day, as an integer between 0 and 23.
day
The day of the month, as an integer between 1 and 31.
month
The month of the year, as an integer between 1 and 12.
year
The year, an integer typically greater than 1900.
dow
The day of week, as an integer between 0 and 6, where 0 stands for Sunday.
dst
t if daylight savings time is effect, otherwise nil.
zone
An integer indicating the time zone, as the number of seconds east of Greenwich.

Common Lisp Note: Common Lisp has different meanings for dow and zone.

Function: encode-time seconds minutes hour day month year &optional zone
This function is the inverse of decode-time. It converts seven items of calendrical data into a time value. For the meanings of the arguments, see the table above under decode-time.

Year numbers less than 100 are not treated specially. If you want them to stand for years above 1900, or years above 2000, you must alter them yourself before you call encode-time.

The optional argument zone defaults to the current time zone and its daylight savings time rules. If specified, it can be either a list (as you would get from current-time-zone), a string as in the TZ environment variable, or an integer (as you would get from decode-time). The specified zone is used without any further alteration for daylight savings time.

If you pass more than seven arguments to encode-time, the first six are used as seconds through year, the last argument is used as zone, and the arguments in between are ignored. This feature makes it possible to use the elements of a list returned by decode-time as the arguments to encode-time, like this:

 
(apply 'encode-time (decode-time ...))

You can perform simple date arithmetic by using out-of-range values for the seconds, minutes, hour, day, and month arguments; for example, day 0 means the day preceding the given month.

The operating system puts limits on the range of possible time values; if you try to encode a time that is out of range, an error results.


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