New version of mu_tm2time function.

The previous version of mu_tm2time incorrectly handled dates within
the DST period.

+/* GNU mailutils - a suite of utilities for electronic mail
+   Copyright (C) 1999, 2000, 2001, 2002 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Library Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.  */
+
+#include <mailutils/mutil.h>
+
+#define SECS_PER_DAY 86400
+#define ADJUSTMENT -719162L
+
+static time_t
+jan1st (int year)
+{
+  year--;               /* Do not consider the current year */
+  return  year*365L
+               + year/4L    /* Years divisible by 4 are leap years */
+               + year/400L  /* Years divisible by 400 are always leap years */
+               - year/100L; /* Years divisible by 100 but not 400 aren't */
+}
+
+static int  month_start[]=
+    {    0, 31, 59,  90, 120, 151, 181, 212, 243, 273, 304, 334, 365 };
+     /* Jan Feb Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec
+         31  28  31   30   31   30   31   31   30   31   30   31
+     */
+
+#define leap_year(y) ((y) % 4 == 0 && (y) % 100 != 0 || (y) % 400 == 0)
+
+static int
+dayofyear (time_t *pday, int year, int month, int day)
+{
+  int  leap, month_days;
+
+  if (year < 0 || month < 0 || month > 11)
+    return -1;
+    
+  leap = leap_year (year);
+  
+  month_days = month_start[month + 1] - month_start[month]
+               + ((month == 2) ? leap : 0);
+
+  if (day < 0 || day > month_days)
+    return -1;  /* Illegal Date */
+
+  if (month <= 2)
+    leap = 0;
+
+  *pday = month_start[month] + day + leap;
+  return 0;
+}
+
+
+/* Convert struct tm into time_t, taking into account timezone offset. */
+/* FIXME: It does not take DST into account */
+time_t
+mu_tm2time (struct tm *tm, mu_timezone *tz)
+{
+  time_t t;
+  
+  if (dayofyear (&t, tm->tm_year, tm->tm_mon, tm->tm_mday - 1))
+    return -1;
+  t = (t + ADJUSTMENT + jan1st (1900 + tm->tm_year)) * SECS_PER_DAY
+            + (tm->tm_hour * 60 + tm->tm_min) * 60 + tm->tm_sec
+      - tz->utc_offset;
+  return t;
+}
+
+/* Convert time 0 at UTC to our localtime, that tells us the offset
+   of our current timezone from UTC. */
+time_t
+mu_utc_offset (void)
+{
+  time_t t = 0;
+  struct tm *tm = gmtime (&t);
+
+  return - mktime (tm);
+}
+
+static const char *months[] =
+{
+  "Jan", "Feb", "Mar", "Apr", "May", "Jun",
+  "Jul", "Aug", "Sep", "Oct", "Nov", "Dec", NULL
+};
+
+static const char *wdays[] =
+{
+  "Sun", "Mon", "Teu", "Wed", "Thr", "Fri", "Sat", NULL
+};
+
+int
+mu_parse_imap_date_time (const char **p, struct tm *tm, mu_timezone *tz)
+{
+  int year, mon, day, hour, min, sec;
+  char zone[6] = "+0000";	/* ( "+" / "-" ) hhmm */
+  char month[5] = "";
+  int hh = 0;
+  int mm = 0;
+  int sign = 1;
+  int scanned = 0, scanned3;
+  int i;
+  int tzoffset;
+
+  day = mon = year = hour = min = sec = 0;
+
+  memset (tm, 0, sizeof (*tm));
+
+  switch (sscanf (*p,
+		  "%2d-%3s-%4d%n %2d:%2d:%2d %5s%n",
+		  &day, month, &year, &scanned3, &hour, &min, &sec, zone,
+		  &scanned))
+    {
+    case 3:
+      scanned = scanned3;
+      break;
+    case 7:
+      break;
+    default:
+      return -1;
+    }
+
+  tm->tm_sec = sec;
+  tm->tm_min = min;
+  tm->tm_hour = hour;
+  tm->tm_mday = day;
+
+  for (i = 0; i < 12; i++)
+    {
+      if (strncasecmp (month, months[i], 3) == 0)
+	{
+	  mon = i;
+	  break;
+	}
+    }
+  tm->tm_mon = mon;
+  tm->tm_year = (year > 1900) ? year - 1900 : year;
+  tm->tm_yday = 0;		/* unknown. */
+  tm->tm_wday = 0;		/* unknown. */
+#if HAVE_STRUCT_TM_TM_ISDST
+  tm->tm_isdst = -1;		/* unknown. */
+#endif
+
+  hh = (zone[1] - '0') * 10 + (zone[2] - '0');
+  mm = (zone[3] - '0') * 10 + (zone[4] - '0');
+  sign = (zone[0] == '-') ? -1 : +1;
+  tzoffset = sign * (hh * 60 * 60 + mm * 60);
+
+#if HAVE_STRUCT_TM_TM_GMTOFF
+  tm->tm_gmtoff = tzoffset;
+#endif
+
+  if (tz)
+    {
+      tz->utc_offset = tzoffset;
+      tz->tz_name = NULL;
+    }
+
+  *p += scanned;
+
+  return 0;
+}
+
+/* "ctime" format is: Thu Jul 01 15:58:27 1999, with no trailing \n.  */
+int
+mu_parse_ctime_date_time (const char **p, struct tm *tm, mu_timezone * tz)
+{
+  int wday = 0;
+  int year = 0;
+  int mon = 0;
+  int day = 0;
+  int hour = 0;
+  int min = 0;
+  int sec = 0;
+  int n = 0;
+  int i;
+  char weekday[5] = "";
+  char month[5] = "";
+
+  if (sscanf (*p, "%3s %3s %2d %2d:%2d:%2d %d%n\n",
+	weekday, month, &day, &hour, &min, &sec, &year, &n) != 7)
+    return -1;
+
+  *p += n;
+
+  for (i = 0; i < 7; i++)
+    {
+      if (strncasecmp (weekday, wdays[i], 3) == 0)
+	{
+	  wday = i;
+	  break;
+	}
+    }
+
+  for (i = 0; i < 12; i++)
+    {
+      if (strncasecmp (month, months[i], 3) == 0)
+	{
+	  mon = i;
+	  break;
+	}
+    }
+
+  if (tm)
+    {
+      memset (tm, 0, sizeof (struct tm));
+
+      tm->tm_sec = sec;
+      tm->tm_min = min;
+      tm->tm_hour = hour;
+      tm->tm_mday = day;
+      tm->tm_wday = wday;
+      tm->tm_mon = mon;
+      tm->tm_year = (year > 1900) ? year - 1900 : year;
+#ifdef HAVE_STRUCT_TM_TM_ISDST
+      tm->tm_isdst = -1;	/* unknown. */
+#endif
+    }
+
+  /* ctime has no timezone information, set tz to UTC if they ask. */
+  if (tz)
+    memset (tz, 0, sizeof (struct mu_timezone));
+
+  return 0;
+}