Re: [9front] Date and time handling.

[ori@eigenstate.org]

> well done!
> 
> i tried to do this years ago whilst writing an ical parser - and gave up in horror.
> 
> -Steve
> 
> 
>> 
>> The formatted draft of the manpage is below, for comments and
>> thoughts:
>> 

After some experimentation and discussion, the API was tweaked a bit.
First, the formatting code was rewritten. Instead of using Go style
format strings, I ended up going with moment.js[1] style format strings,
with some tweaks and simplifications. There were too many edge cases
in parsing the date and guessing at what the numbers meant for an
elegant parser.

Second, loading the timezone was made explicit. There were a number
of errors that were easily ignored when loading invalid timezones,
and timezone naming is a huge mess -- so making loading the timezone
explicit. So, now you'd write:

	if((local = tmgetzone("local")) == nil)
		sysfatal("get zone: %r\n");
	if(tmtime(&tm, 1586218416ll, local) == nil)
		sysfatal("now: %r");
	tmfmt(buf, sizeof(buf), "YYYY/MM/DD hh:mm:ss", &tm);

The updated manpage is below:

     TMDATE(2)                                               TMDATE(2)

     NAME
          tmnow, tmtime, tmstime, tmshiftzone, tmparse, tmfmt, tmnorm,
          - convert date and time

     SYNOPSIS
          #include <u.h>
          #include <libc.h>

          typedef struct Tmd Tmd;
          struct {
                vlong    abs; /* seconds since Jan 1 1970, GMT */
                int sec; /* seconds (range 0..59) */
                int min; /* minutes (0..59) */
                int hour;     /* hours (0..23) */
                int mday;     /* day of the month (1..31) */
                int mon; /* month of the year (0..11) */
                int year;     /* year A.D. - 1900 */
                int wday;     /* day of week (0..6, Sunday = 0) */
                int yday;     /* day of year (0..365) */
                char     zone[];   /* time zone name */
                int tzoff;    /* time   zone delta from GMT */
          };

          Tzone *tmgetzone(char *name);
          Tm    *tmnow(Tm *tm, char *tz);
          Tm    *tmtime(Tm *tm, vlong abs, Tzone *tz);
          Tm    *tmstime(Tm *tm, vlong sec, Tzone *tz);
          Tm    *tmshiftzone(Tm *dst, Tm *src, Tzone *tz);
          Tm    *tmparse(char *fmt, char *tm, Tzone *zone, Tm *dst);
          int   tmfmt(char *buf, usize nbuf, char *fmt, Tm *tm);
          void tmnorm(Tm *tm);
          void tmfmtinstall(char *fmt);

     DESCRIPTION
          This family of functions handles simple date and time manpu-
          lation.  Times are represented as an absolute instant in
          time, combined with a time zone.

          Time zones are loaded by as name.  They can be specified as
          the abbreviated timezone name, the full timezone name, the
          path to a timezone file, or an absolute offset in the HHMM
          form.

          When given as a timezone, any instant-dependent adjustments
          such as leap seconds and daylight savings time will be
          applied to the derived fields of struct tm, but will not
          affect the absolute time.  The time zone name local always
          refers to the time in /env/timezone.  The nil timezone
          always refers to GMT.

          Tmgetzone loads a timezone by name. The returned timezone is
          cached for the lifetime of the program, and should not be
          freed.  Loading a timezone repeatedly by name loads from the
          cache, and does not leak.

          Tmnow gets the current time of day in the requested time
          zone.

          Tmtime converts the millisecond-resolution timestamp 'abs'
          into a Tm struct in the requested timezone.

          Tmstime is identical to tmtime, but accepts the time in sec-
          onds.

          Tmshiftzone moves a time from one timezone to another, doing
          the appropriate conversions.

          Tmparse parses a time from a string according to the format
          argument. The result is returned in the timezone requested.
          If there is a timezone in the date, then we tzshift to the
          local timezone.

          The format argument takes contains zero or more of the fol-
          lowing components:

          Y, YY, YYYY
               Represents the year.  YY prints the year in 2 digit
               form.

          M, MM, MMM, MMMM
               The month of the year, in unpadded numeric, padded
               numeric, short name, or long name, respectively.

          D, DD
               The day of month in unpadded or padded numeric form,
               respectively.

          W, WW
               The day of week in short or long name form, respec-
               tively.

          h, hh
               The hour in unpadded or padded form, respectively

          m, mm
               The minute in unpadded or padded form, respectively

          s, ss
               The second in unpadded or padded form, respectively

          z, Z, ZZ
               The timezone in named, [+-]HHMM and [+-]HH:MM form,
               respectively

          a, A Lower and uppercase 'am' and 'pm' specifiers, respec-
               tively.

          [...]
               Quoted text, copied directly to the output.

          Any characters not specified above are copied directly to
          output, without modification.

          If the format argument is nil, it makes an attempt to parse
          common human readable date formats.  These formats include
          ISO-8601,RFC-3339 and RFC-2822 dates.

          Tmfmt formats a Tm struct according to the format fmt. If
          fmt is nil, we format as in ctime(2). At most characters are
          written into buf, including the terminator.  The format is
          identical to tmparse.

          When parsing, any amount of whitespace is treated as a sin-
          gle token.  All string matches are case insensitive, and
          zero padding is optional.

          Tmrecalc takes a manually adjusted Tm structure, and recal-
          culates the absolute time from the year, mon, mday, hr, min
          and sec fields. Other fields are ignored.  This recalcula-
          tion respects the time zone stored in struct tm.  Out of
          range values are wrapped. For example, December 32nd becomes
          January 1st.

          Tmfmtinstall installs a time format specifier %T. The time
          format behaves as in tmfmt

     Examples
          All examples assume tmfmtinstall has been called.

          Get the current date in the local timezone, GMT, and
          US_Pacific time. Print it using the default format.

          Tm t;
          print("local: %T0, tmnow(&t, "local"));
          print("gmt: %T0, tmnow(&t, "GMT"));
          print("eastern: %T0, tmnow(&t, "US_Pacific"));

          Compare if two times are the same, regardless of timezone.

          Tm a, b;

          tmparse(&a, nil, "Tue Dec 10 12:36:00 PST 2019");
          tmparse(&b, nil, "Tue Dec 10 15:36:00 EST 2019");
          if(a.abs == b.abs)
               print("same0);
          else
               print("different0);

          Add a day to two times. Because we picked daylight savings
          time to adjust over, only 23 hours are added.

          Tm t;
          tmparse(&t, "W MMM D hh:mm:ss z YYYY, "Sun Nov 2 13:11:11 PST 2019");
          tm.day++;
          tmrecalc(&t);
          print("%T", &t); /*  Mon Nov 3 13:11:11 PST 2019 */