.\"     $NetBSD: parsedate.3,v 1.24 2017/03/22 18:17:42 kre Exp $
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Christos Zoulas.
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.Dd March 22, 2017
.Dt PARSEDATE 3
.Os
.Sh NAME
.Nm parsedate
.Nd date parsing function
.Sh LIBRARY
.Lb libutil
.Sh SYNOPSIS
.In util.h
.Ft time_t
.Fn parsedate "const char *datestr" "const time_t *time" "const int *tzoff"
.Sh DESCRIPTION
The
.Fn parsedate
function parses a datetime from
.Ar datestr
described in English relative to an optional
.Ar time
point,
and an optional timezone offset (in minutes behind/west of UTC)
specified in
.Ar tzoff .
If
.Ar time
is
.Dv NULL
then the current time is used.
If
.Ar tzoff
is
.Dv NULL ,
then the current time zone is used.
.Pp
The
.Ar datestr
is a sequence of white-space separated items.
The white-space is optional if the concatenated items are not ambiguous.
An empty
.Ar datestr
is equivalent to midnight today (the beginning of this day).
.Pp
The following words have the indicated numeric meanings:
.Dv last =
\-1,
.Dv this =
0,
.Dv first , next ,
or
.Dv one =
1,
.Dv second
is unused so that it is not confused with
.Dq seconds ,
.Dv two =
2,
.Dv third
or
.Dv three =
3,
.Dv fourth
or
.Dv four =
4,
.Dv fifth
or
.Dv five  =
5,
.Dv sixth
or
.Dv six  =
6,
.Dv seventh
or
.Dv seven =
7,
.Dv eighth
or
.Dv eight =
8,
.Dv ninth
or
.Dv nine =
9,
.Dv tenth
or
.Dv ten =
10,
.Dv eleventh
or
.Dv eleven =
11,
.Dv twelfth
or
.Dv twelve =
12.
.Pp
The following words are recognized in English only:
.Dv AM ,
.Dv PM ,
.Dv a.m. ,
.Dv p.m. ,
.Dv midnight ,
.Dv mn ,
.Dv noon .
.Pp
The months:
.Dv january ,
.Dv february ,
.Dv march ,
.Dv april ,
.Dv may ,
.Dv june ,
.Dv july ,
.Dv august ,
.Dv september ,
.Dv october ,
.Dv november ,
.Dv december ,
and common abbreviations for them.
.Pp
The days of the week:
.Dv sunday ,
.Dv monday ,
.Dv tuesday ,
.Dv wednesday ,
.Dv thursday ,
.Dv friday ,
.Dv saturday ,
and common abbreviations for them.
.Pp
Time units:
.Dv year ,
.Dv month ,
.Dv fortnight ,
.Dv week ,
.Dv day ,
.Dv hour ,
.Dv minute ,
.Dv min ,
.Dv second ,
.Dv sec ,
.Dv tomorrow ,
.Dv yesterday .
.Pp
Timezone names:
.Dv gmt (+0000) ,
.Dv ut (+0000) ,
.Dv utc (+0000) ,
.Dv wet (+0000) ,
.Dv bst (+0100) ,
.Dv wat (-0100) ,
.Dv at (-0200) ,
.Dv nft (-0330) ,
.Dv nst (-0330) ,
.Dv ndt (-0230) ,
.Dv ast (-0400) ,
.Dv adt (-0300) ,
.Dv est (-0500) ,
.Dv edt (-0400) ,
.Dv cst (-0600) ,
.Dv cdt (-0500) ,
.Dv mst (-0700) ,
.Dv mdt (-0600) ,
.Dv pst (-0800) ,
.Dv pdt (-0700) ,
.Dv yst (-0900) ,
.Dv ydt (-0800) ,
.Dv hst (-1000) ,
.Dv hdt (-0900) ,
.Dv cat (-1000) ,
.Dv ahst (-1000) ,
.Dv nt (-1100) ,
.Dv idlw (-1200) ,
.Dv cet (+0100) ,
.Dv met (+0100) ,
.Dv mewt (+0100) ,
.Dv mest (+0200) ,
.Dv swt (+0100) ,
.Dv sst (+0200) ,
.Dv fwt (+0100) ,
.Dv fst (+0200) ,
.Dv eet (+0200) ,
.Dv bt (+0300) ,
.Dv it (+0330) ,
.Dv zp4 (+0400) ,
.Dv zp5 (+0500) ,
.Dv ist (+0550) ,
.Dv zp6 (+0600) ,
.Dv ict (+0700) ,
.Dv wast (+0800) ,
.Dv wadt (+0900) ,
.Dv awst (+0800) ,
.Dv awdt (+0900) ,
.Dv cct (+0800) ,
.Dv sgt (+0800) ,
.Dv hkt (+0800) ,
.Dv jst (+0900) ,
.Dv cast (+0930) ,
.Dv cadt (+1030) ,
.Dv acst (+0930) ,
.Dv acst (+1030) ,
.Dv east (+1000) ,
.Dv eadt (+1100) ,
.Dv aest (+1000) ,
.Dv aedt (+1100) ,
.Dv gst (+1000) ,
.Dv nzt (+1200) ,
.Dv nzst (+1200) ,
.Dv nzdt (+1300) ,
.Dv idle (+1200) .
.Pp
The timezone names specify an offset from Coordinated Universal Time (UTC)
and do not imply validating the time/date to be reasonable in any zone
that happens to use the abbreviation specified.
.Pp
A variety of unambiguous dates are recognized:
.Bl -tag -compact -width "20 Jun 1994"
.It 9/10/69
For years between 70-99 we assume 1900+ and for years between 0-69
we assume 2000+.
.It 2006-11-17
An ISO-8601 date.
.It 69-09-10
The year in an ISO-8601 date is always taken literally,
so this is the year 69, not 2069.
.It 10/1/2000
October 1, 2000; the common, but bizarre, US format.
.It 20 Jun 1994
.It 23jun2001
.It 1-sep-06
Other common abbreviations.
.It 1/11
The year can be omitted.
This is the US month/day format.
.El
.Pp
Standard e-mail (RFC822, RFC2822, etc)
formats and the output from
.Xr date 1 ,
and
.Xr asctime 3
are all supported as input.
.Pp
As well as times:
.Bl -tag -compact -width 12:11:01.000012
.It 10:01
.It 10:12pm
.It 12:11:01.000012
.It 12:21-0500
.El
Fractions of seconds (after a decimal point) are parsed, but ignored.
.Pp
Relative items are also supported:
.Bl -tag -compact -width "this thursday"
.It -1 month
.It last friday
.It one week ago
.It this thursday
.It next sunday
.It +2 years
.El
.Pp
Note that, as a special case for
.Dv midnight
with the name of a day only,
.Dq "midnight tuesday"
implies 00:00 at the beginning of Tuesday, whereas
.Dq "Sat mn"
implies 00:00 at the end of Saturday (i.e. early Sunday morning.)
.Pp
Seconds since epoch, UTC, (also known as UNIX time) are also supported:
.Bl -tag -compact -width "@735275209"
.It @735275209
Tue Apr 20 03:06:49 UTC 1993
.El
provided that the value given is within the range
that can be represented as a
.Va "struct tm" .
Negative values
(times before the epoch)
are permitted, but no other significant data.
.Pp
Text in
.Ar datestr
enclosed in parentheses
.Ql \&(
and
.Ql \&)
is treated as a comment, and ignored.
Parentheses nest (the comment ends when there have
been the same number of closing parentheses as there
were opening parentheses.)
There is no escape character in comments,
.Ql \&)
always ends
(or decreases the nesting level of)
the comment.
.Sh RETURN VALUES
.Fn parsedate
returns the number of seconds passed since,
or before (if negative,)
the Epoch, or
.Dv \-1
if the date could not be parsed properly.
A non-error result of
.Dv \-1
can be distinguished from an error by setting
.Va errno
to
.Dv 0
before calling
.Fn parsedate ,
and checking the value of
.Va errno
afterwards.
.Sh ENVIRONMENT
If the
.Ar tzoff
parameter is given as
.Dv NULL ,
then:
.Bl -tag -width iTZ
.It Ev TZ
The timezone to which the input is relative,
when no zone information is otherwise specified in the
.Ar datestr
input.
.El
.Sh SEE ALSO
.Xr date 1 ,
.Xr touch 1 ,
.Xr errno 2 ,
.Xr ctime 3 ,
.\" WTF ????  eeprom(8)!!  Why?  Just because it calls this function?  Weird!
.Xr eeprom 8
.Sh HISTORY
The parser used in
.Fn parsedate
was originally written by Steven M. Bellovin while at the University
of North Carolina at Chapel Hill.
It was later tweaked by a couple of people on Usenet.
Completely overhauled by Rich $alz and Jim Berets in August, 1990.
.Pp
The
.Fn parsedate
function first appeared in
.Nx 4.0 .
.Sh BUGS
.Bl -tag -compact -width 1
.It 1
The
.Fn parsedate
function is not re-entrant or thread-safe.
.It 2
The
.Fn parsedate
function assumes years less than 0 mean \(mi
.Fa year ,
and in non ISO formats,
that years less than 70 mean 2000 +
.Fa year ,
otherwise
years less than 100 mean 1900 +
.Fa year .
.It 3
The
.Fn parsedate
function accepts
.Dq "12 am"
where
.Dq "12 midnight"
is correct, and similarly
.Dq "12 pm"
for
.Dq "12 noon" .
The correct forms are also accepted.
.It 4
There are various weird cases that are hard to explain,
but are nevertheless considered correct.
.It 5
It is very hard to specify years BC,
and in any case,
conversions of times before the
commencement of the modern Gregorian calendar
(when that occurred depends upon location,
but late 16th century is a rough guide)
are suspicious at best,
and depending upon context,
often just plain wrong.
.It 6
Despite what is stated above,
.Dq next
is actually 2.
The input
.Dq "next January" ,
instead of producing a timestamp for January of the
following year, produces one for January 2nd, of the
current year.
Use caution with
.Dq next
it rarely does what humans expect.
.El