/* gettime.c - Wrapper for time functions
 * Copyright (C) 1998, 2002, 2007, 2011 Free Software Foundation, Inc.
 *
 * This file is part of GnuPG.
 *
 * This file is free software; you can redistribute it and/or modify
 * it under the terms of either
 *
 *   - the GNU Lesser General Public License as published by the Free
 *     Software Foundation; either version 3 of the License, or (at
 *     your option) any later version.
 *
 * or
 *
 *   - the GNU General Public License as published by the Free
 *     Software Foundation; either version 2 of the License, or (at
 *     your option) any later version.
 *
 * or both in parallel, as here.
 *
 * This file 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, see <https://www.gnu.org/licenses/>.
 */

#include <config.h>
#include <stdlib.h>
#include <time.h>
#include <ctype.h>
#ifdef HAVE_LOCALE_H
#include <locale.h>
#endif
#ifdef HAVE_LANGINFO_H
#include <langinfo.h>
#endif
#ifdef HAVE_W32_SYSTEM
# define WIN32_LEAN_AND_MEAN
# include <windows.h>
#endif /*!HAVE_W32_SYSTEM*/
#include <stdint.h>    /* We use uint64_t.  */

#include "util.h"
#include "i18n.h"
#include "gettime.h"

#ifdef HAVE_W32_SYSTEM
#include <windows.h>
#endif

#ifdef HAVE_UNSIGNED_TIME_T
# define IS_INVALID_TIME_T(a) ((a) == (time_t)(-1))
#else
  /* Error or 32 bit time_t and value after 2038-01-19.  */
# define IS_INVALID_TIME_T(a) ((a) < 0)
#endif


static unsigned long timewarp;
static enum { NORMAL = 0, FROZEN, FUTURE, PAST } timemode;

/* Correction used to map to real Julian days. */
#define JD_DIFF 1721060L



/*
  timegm() is a GNU function that might not be available everywhere.
  It's basically the inverse of gmtime() - you give it a struct tm,
  and get back a time_t.  It differs from mktime() in that it handles
  the case where the struct tm is UTC and the local environment isn't.

  Note, that this replacement implementation might not be thread-safe!

  Some BSDs don't handle the putenv("foo") case properly, so we use
  unsetenv if the platform has it to remove environment variables.
*/
#ifndef HAVE_TIMEGM
time_t
timegm (struct tm *tm)
{
#ifdef HAVE_W32_SYSTEM
  uint64_t val = timegm_u64 (tm);
  if (val == (uint64_t)(-1))
    return (time_t)(-1);
  return (time_t)val;
#else /* (Non thread safe implementation!) */

  time_t answer;
  char *zone;

  zone=getenv("TZ");
  putenv("TZ=UTC");
  tzset();
  answer=mktime(tm);
  if(zone)
    {
      static char *old_zone;

      if (!old_zone)
        {
          old_zone = malloc(3+strlen(zone)+1);
          if (old_zone)
            {
              strcpy(old_zone,"TZ=");
              strcat(old_zone,zone);
            }
	}
      if (old_zone)
        putenv (old_zone);
    }
  else
    gnupg_unsetenv("TZ");

  tzset();
  return answer;
#endif
}
#endif /*!HAVE_TIMEGM*/


/* Version of the GNU timegm which returns an unsigned 64 bit integer
 * instead of the usually signed time_t.  On error (uint64_t)(-1) is
 * returned.  This function is mostly here because on 32 bit Windows
 * we have an internal API to get the system time even after
 * 2023-01-19.  For 32 bit Unix we need to suffer from the too short
 * time_t and no system function to construct the time from a tm.  */
uint64_t
timegm_u64 (struct tm *tm)
{
#ifdef HAVE_W32_SYSTEM
  /* This one is thread safe.  */
  SYSTEMTIME st;
  FILETIME ft;
  unsigned long long cnsecs;

  st.wYear   = tm->tm_year + 1900;
  st.wMonth  = tm->tm_mon  + 1;
  st.wDay    = tm->tm_mday;
  st.wHour   = tm->tm_hour;
  st.wMinute = tm->tm_min;
  st.wSecond = tm->tm_sec;
  st.wMilliseconds = 0; /* Not available.  */
  st.wDayOfWeek = 0;    /* Ignored.  */

  /* System time is UTC thus the conversion is pretty easy.  */
  if (!SystemTimeToFileTime (&st, &ft))
    {
      gpg_err_set_errno (EINVAL);
      return (uint64_t)(-1);
    }

  cnsecs = (((unsigned long long)ft.dwHighDateTime << 32)
            | ft.dwLowDateTime);
  cnsecs -= 116444736000000000ULL; /* The filetime epoch is 1601-01-01.  */
  return (uint64_t)(cnsecs / 10000000ULL);

#else /*Unix*/

  time_t t = timegm (tm);
  if (t == (time_t)(-1))
    return (uint64_t)(-1);
  if ((int64_t)t < 0)
    return (uint64_t)(-1);
  return (uint64_t)t;

#endif /*Unix*/
}


/* Wrapper for the time(3).  We use this here so we can fake the time
   for tests */
time_t
gnupg_get_time (void)
{
  time_t current = time (NULL);
  if (current == (time_t)(-1))
    log_fatal ("time() failed\n");

  if (timemode == NORMAL)
    return current;
  else if (timemode == FROZEN)
    return timewarp;
  else if (timemode == FUTURE)
    return current + timewarp;
  else
    return current - timewarp;
}


/* Wrapper around gmtime_r.

   On systems without gmtime_r this implementation works within gnupg
   because we use only one thread a time.  FIXME: An independent
   library may use gmtime in one of its own thread (or via
   npth_enter/npth_leave) - in this case we run into a problem.  The
   solution would be to use a mutex here.  */
struct tm *
gnupg_gmtime (const time_t *timep, struct tm *result)
{
#ifdef HAVE_GMTIME_R
  return gmtime_r (timep, result);
#else
  struct tm *tp;

  tp = gmtime (timep);
  if (tp)
    memcpy (result, tp, sizeof *result);
  return tp;
#endif
}


/* Return the current time (possibly faked) in ISO format. */
void
gnupg_get_isotime (gnupg_isotime_t timebuf)
{
  time_t atime = gnupg_get_time ();
  struct tm *tp;
  struct tm tmbuf;

  tp = gnupg_gmtime (&atime, &tmbuf);
  if (!tp)
    *timebuf = 0;
  else
    snprintf (timebuf, 16, "%04d%02d%02dT%02d%02d%02d",
              1900 + tp->tm_year, tp->tm_mon+1, tp->tm_mday,
              tp->tm_hour, tp->tm_min, tp->tm_sec);
}


/* Set the time to NEWTIME so that gnupg_get_time returns a time
   starting with this one.  With FREEZE set to 1 the returned time
   will never change.  Just for completeness, a value of (time_t)-1
   for NEWTIME gets you back to reality.  Note that this is obviously
   not thread-safe but this is not required. */
void
gnupg_set_time (time_t newtime, int freeze)
{
  time_t current = time (NULL);

  if ( newtime == (time_t)-1 || current == newtime)
    {
      timemode = NORMAL;
      timewarp = 0;
    }
  else if (freeze)
    {
      timemode = FROZEN;
      timewarp = newtime == (time_t)-1 ? current : newtime;
    }
  else if (newtime > current)
    {
      timemode = FUTURE;
      timewarp = newtime - current;
    }
  else
    {
      timemode = PAST;
      timewarp = current - newtime;
    }
}

/* Returns true when we are in timewarp mode */
int
gnupg_faked_time_p (void)
{
  return timemode;
}


/* This function is used by gpg because OpenPGP defines the timestamp
   as an unsigned 32 bit value. */
u32
make_timestamp (void)
{
  time_t t = gnupg_get_time ();
  return (u32)t;
}


/* Specialized version of atoi which returns an u32 instead of an int
 * and caps the result at 2^32-2.  Leading white space is skipped,
 * scanning stops at at the first non-convertable byte.  Note that we
 * do not cap at 2^32-1 because that value is often used as error
 * return. */
u32
scan_secondsstr (const char *string)
{
  uint64_t value = 0;

  while (*string == ' ' || *string == '\t')
    string++;
  for (; *string >= '0' && *string <= '9'; string++)
    {
      value *= 10;
      value += atoi_1 (string);
      if (value >= (u32)(-1))
        return (u32)(-1) - 1;
    }
  return (u32)value;
}


/****************
 * Scan a date string and return a timestamp.
 * The only supported format is "yyyy-mm-dd"
 * Returns 0 for an invalid date.
 */
u32
scan_isodatestr( const char *string )
{
    int year, month, day;
    struct tm tmbuf;
    time_t stamp;
    int i;

    if( strlen(string) != 10 || string[4] != '-' || string[7] != '-' )
	return 0;
    for( i=0; i < 4; i++ )
	if( !digitp (string+i) )
	    return 0;
    if( !digitp (string+5) || !digitp(string+6) )
	return 0;
    if( !digitp(string+8) || !digitp(string+9) )
	return 0;
    year = atoi(string);
    month = atoi(string+5);
    day = atoi(string+8);
    /* some basic checks */
    if( year < 1970 || month < 1 || month > 12 || day < 1 || day > 31 )
	return 0;
    memset( &tmbuf, 0, sizeof tmbuf );
    tmbuf.tm_mday = day;
    tmbuf.tm_mon = month-1;
    tmbuf.tm_year = year - 1900;
    tmbuf.tm_isdst = -1;
    stamp = mktime( &tmbuf );
    if( stamp == (time_t)-1 )
      {
        /* mktime did not work.  Construct an ISO timestring for noon
         * of the given day instead.  We keep the use of mktime for 64
         * bit system to limit the risk of regressions. */
        gnupg_isotime_t isobuf;
        uint64_t tmp64;

        snprintf (isobuf, 16, "%04d%02d%02dT120000", year, month, day);
        tmp64 = isotime2epoch_u64 (isobuf);
        if (tmp64 == (uint64_t)(-1))
          return 0;  /* Error.  */
        if (tmp64 >= (u32)(-1))
          return 0;   /* Error.  */
        return (u32)tmp64;
      }
    return stamp;
}


int
isotime_p (const char *string)
{
  const char *s;
  int i;

  if (!*string)
    return 0;
  for (s=string, i=0; i < 8; i++, s++)
    if (!digitp (s))
      return 0;
  if (*s != 'T')
      return 0;
  for (s++, i=9; i < 15; i++, s++)
    if (!digitp (s))
      return 0;
  if (*s == 'Z')
    s++;
  if ( !(!*s || (isascii (*s) && isspace(*s)) || *s == ':' || *s == ','))
    return 0;  /* Wrong delimiter.  */

  return 1;
}


/* Scan a string and return true if the string represents the human
   readable format of an ISO time.  This format is:
      yyyy-mm-dd[ hh[:mm[:ss]]]
   Scanning stops at the second space or at a comma.  If DATE_ONLY is
   true the time part is not expected and the scanning stops at the
   first space or at a comma. */
int
isotime_human_p (const char *string, int date_only)
{
  const char *s;
  int i;

  if (!*string)
    return 0;
  for (s=string, i=0; i < 4; i++, s++)
    if (!digitp (s))
      return 0;
  if (*s != '-')
    return 0;
  s++;
  if (!digitp (s) || !digitp (s+1) || s[2] != '-')
    return 0;
  i = atoi_2 (s);
  if (i < 1 || i > 12)
    return 0;
  s += 3;
  if (!digitp (s) || !digitp (s+1))
    return 0;
  i = atoi_2 (s);
  if (i < 1 || i > 31)
    return 0;
  s += 2;
  if (!*s || *s == ',')
    return 1; /* Okay; only date given.  */
  if (!spacep (s))
    return 0;
  if (date_only)
    return 1; /* Okay; only date was requested.  */
  s++;
  if (spacep (s))
    return 1; /* Okay, second space stops scanning.  */
  if (!digitp (s) || !digitp (s+1))
    return 0;
  i = atoi_2 (s);
  if (i < 0 || i > 23)
    return 0;
  s += 2;
  if (!*s || *s == ',')
    return 1; /* Okay; only date and hour given.  */
  if (*s != ':')
    return 0;
  s++;
  if (!digitp (s) || !digitp (s+1))
    return 0;
  i = atoi_2 (s);
  if (i < 0 || i > 59)
    return 0;
  s += 2;
  if (!*s || *s == ',')
    return 1; /* Okay; only date, hour and minute given.  */
  if (*s != ':')
    return 0;
  s++;
  if (!digitp (s) || !digitp (s+1))
    return 0;
  i = atoi_2 (s);
  if (i < 0 || i > 60)
    return 0;
  s += 2;
  if (!*s || *s == ',' || spacep (s))
    return 1; /* Okay; date, hour and minute and second given.  */

  return 0; /* Unexpected delimiter.  */
}

/* Convert a standard isotime or a human readable variant into an
   isotime structure.  The allowed formats are those described by
   isotime_p and isotime_human_p.  The function returns 0 on failure
   or the length of the scanned string on success.  */
size_t
string2isotime (gnupg_isotime_t atime, const char *string)
{
  gnupg_isotime_t dummyatime;

  if (!atime)
    atime = dummyatime;

  atime[0] = 0;
  if (isotime_p (string))
    {
      memcpy (atime, string, 15);
      atime[15] = 0;
      return 15;
    }
  if (!isotime_human_p (string, 0))
    return 0;
  atime[0] = string[0];
  atime[1] = string[1];
  atime[2] = string[2];
  atime[3] = string[3];
  atime[4] = string[5];
  atime[5] = string[6];
  atime[6] = string[8];
  atime[7] = string[9];
  atime[8] = 'T';
  memset (atime+9, '0', 6);
  atime[15] = 0;
  if (!spacep (string+10))
    return 10;
  if (spacep (string+11))
    return 11; /* As per def, second space stops scanning.  */
  atime[9] = string[11];
  atime[10] = string[12];
  if (string[13] != ':')
    return 13;
  atime[11] = string[14];
  atime[12] = string[15];
  if (string[16] != ':')
    return 16;
  atime[13] = string[17];
  atime[14] = string[18];
  return 19;
}


/* Helper for isotime2epoch.  Returns 0 on success. */
static int
isotime_make_tm (const char *string, struct tm *tmbuf)
{
  int year, month, day, hour, minu, sec;

  if (!isotime_p (string))
    return -1;

  year  = atoi_4 (string);
  month = atoi_2 (string + 4);
  day   = atoi_2 (string + 6);
  hour  = atoi_2 (string + 9);
  minu  = atoi_2 (string + 11);
  sec   = atoi_2 (string + 13);

  /* Basic checks.  */
  if (year < 1970 || month < 1 || month > 12 || day < 1 || day > 31
      || hour > 23 || minu > 59 || sec > 61 )
    return -1;

  memset (tmbuf, 0, sizeof *tmbuf);
  tmbuf->tm_sec  = sec;
  tmbuf->tm_min  = minu;
  tmbuf->tm_hour = hour;
  tmbuf->tm_mday = day;
  tmbuf->tm_mon  = month-1;
  tmbuf->tm_year = year - 1900;
  tmbuf->tm_isdst = -1;
  return 0;
}


/* Scan an ISO timestamp and return an Epoch based timestamp.  The
   only supported format is "yyyymmddThhmmss[Z]" delimited by white
   space, nul, a colon or a comma.  Returns (time_t)(-1) for an
   invalid string.  */
time_t
isotime2epoch (const char *string)
{
  struct tm tmbuf;

  if (isotime_make_tm (string, &tmbuf))
    return (time_t)(-1);

  return timegm (&tmbuf);
}


uint64_t
isotime2epoch_u64 (const char *string)
{
  struct tm tmbuf;

  if (isotime_make_tm (string, &tmbuf))
    return (uint64_t)(-1);

  return timegm_u64 (&tmbuf);
}


/* Convert an Epoch time to an iso time stamp. */
void
epoch2isotime (gnupg_isotime_t timebuf, time_t atime)
{
  if (atime == (time_t)(-1))
    *timebuf = 0;
  else
    {
      struct tm *tp;
#ifdef HAVE_GMTIME_R
      struct tm tmbuf;

      tp = gmtime_r (&atime, &tmbuf);
#else
      tp = gmtime (&atime);
#endif
      snprintf (timebuf, 16, "%04d%02d%02dT%02d%02d%02d",
                1900 + tp->tm_year, tp->tm_mon+1, tp->tm_mday,
                tp->tm_hour, tp->tm_min, tp->tm_sec);
    }
}


/* Parse a short ISO date string (YYYY-MM-DD) into a TM structure.
   Returns 0 on success.  */
int
isodate_human_to_tm (const char *string, struct tm *t)
{
  int year, month, day;

  if (!isotime_human_p (string, 1))
    return -1;

  year  = atoi_4 (string);
  month = atoi_2 (string + 5);
  day   = atoi_2 (string + 8);

  /* Basic checks.  */
  if (year < 1970 || month < 1 || month > 12 || day < 1 || day > 31)
    return -1;

  memset (t, 0, sizeof *t);
  t->tm_sec  = 0;
  t->tm_min  = 0;
  t->tm_hour = 0;
  t->tm_mday = day;
  t->tm_mon  = month-1;
  t->tm_year = year - 1900;
  t->tm_isdst = -1;
  return 0;
}


/* Parse the string TIMESTAMP into a time_t.  The string may either be
   seconds since Epoch or in the ISO 8601 format like
   "20390815T143012".  Returns 0 for an empty string or seconds since
   Epoch. Leading spaces are skipped. If ENDP is not NULL, it will
   point to the next non-parsed character in TIMESTRING.

   This function is a copy of
   gpgme/src/conversion.c:_gpgme_parse_timestamp.  If you change it,
   then update the other one too.

   FIXME: Replace users of this function by one of the more modern
          functions or change the return type to u64.
*/
time_t
parse_timestamp (const char *timestamp, char **endp)
{
  /* Need to skip leading spaces, because that is what strtoul does
     but not our ISO 8601 checking code. */
  while (*timestamp && *timestamp== ' ')
    timestamp++;
  if (!*timestamp)
    return 0;

  if (strlen (timestamp) >= 15 && timestamp[8] == 'T')
    {
      struct tm buf;
      int year;

      year = atoi_4 (timestamp);
      if (year < 1900)
        return (time_t)(-1);

      if (endp)
        *endp = (char*)(timestamp + 15);

      /* Fixme: We would better use a configure test to see whether
         mktime can handle dates beyond 2038. */
      if (sizeof (time_t) <= 4 && year >= 2038)
        return (time_t)2145914603; /* 2037-12-31 23:23:23 */

      memset (&buf, 0, sizeof buf);
      buf.tm_year = year - 1900;
      buf.tm_mon = atoi_2 (timestamp+4) - 1;
      buf.tm_mday = atoi_2 (timestamp+6);
      buf.tm_hour = atoi_2 (timestamp+9);
      buf.tm_min = atoi_2 (timestamp+11);
      buf.tm_sec = atoi_2 (timestamp+13);

      return timegm (&buf);
    }
  else
    return (time_t)strtoul (timestamp, endp, 10);
}



u32
add_days_to_timestamp( u32 stamp, u16 days )
{
    return stamp + days*86400L;
}


/****************
 * Return a string with a time value in the form: x Y, n D, n H
 */

const char *
strtimevalue( u32 value )
{
    static char buffer[30];
    unsigned int years, days, hours, minutes;

    value /= 60;
    minutes = value % 60;
    value /= 60;
    hours = value % 24;
    value /= 24;
    days = value % 365;
    value /= 365;
    years = value;

    sprintf(buffer,"%uy%ud%uh%um", years, days, hours, minutes );
    if( years )
	return buffer;
    if( days )
	return strchr( buffer, 'y' ) + 1;
    return strchr( buffer, 'd' ) + 1;
}



/* Return a malloced string with the time elapsed between NOW and
   SINCE.  May return NULL on error. */
char *
elapsed_time_string (time_t since, time_t now)
{
  char *result;
  double diff;
  unsigned long value;
  unsigned int days, hours, minutes, seconds;

  if (!now)
    now = gnupg_get_time ();

  diff = difftime (now, since);
  if (diff < 0)
    return xtrystrdup ("time-warp");

  seconds = (unsigned long)diff % 60;
  value = (unsigned long)(diff / 60);
  minutes = value % 60;
  value /= 60;
  hours = value % 24;
  value /= 24;
  days = value % 365;

  if (days)
    result = xtryasprintf ("%ud%uh%um%us", days, hours, minutes, seconds);
  else if (hours)
    result = xtryasprintf ("%uh%um%us", hours, minutes, seconds);
  else if (minutes)
    result = xtryasprintf ("%um%us", minutes, seconds);
  else
    result = xtryasprintf ("%us", seconds);

  return result;
}


/*
 * Note: this function returns GMT
 */
const char *
strtimestamp (u32 stamp)
{
  static char buffer[11+5];
  struct tm *tp;
  time_t atime = stamp;

  if (IS_INVALID_TIME_T (atime))
    {
      strcpy (buffer, "????" "-??" "-??");
    }
  else
    {
      tp = gmtime( &atime );
      snprintf (buffer, sizeof buffer, "%04d-%02d-%02d",
                1900+tp->tm_year, tp->tm_mon+1, tp->tm_mday );
    }
  return buffer;
}


/*
 * Note: this function returns GMT
 */
const char *
isotimestamp (u32 stamp)
{
  static char buffer[25+5];
  struct tm *tp;
  time_t atime = stamp;

  if (IS_INVALID_TIME_T (atime))
    {
      strcpy (buffer, "????" "-??" "-??" " " "??" ":" "??" ":" "??");
    }
  else
    {
      tp = gmtime ( &atime );
      snprintf (buffer, sizeof buffer, "%04d-%02d-%02d %02d:%02d:%02d",
                1900+tp->tm_year, tp->tm_mon+1, tp->tm_mday,
                tp->tm_hour, tp->tm_min, tp->tm_sec);
    }
  return buffer;
}


/****************
 * Note: this function returns local time
 */
const char *
asctimestamp (u32 stamp)
{
  static char buffer[80];
#if defined (HAVE_STRFTIME) && defined (HAVE_NL_LANGINFO)
  static char fmt[80];
#endif
  struct tm *tp;
  time_t atime = stamp;

  if (IS_INVALID_TIME_T (atime))
    {
      strcpy (buffer, "????" "-??" "-??");
      return buffer;
    }

  tp = localtime( &atime );
#ifdef HAVE_STRFTIME
# if defined(HAVE_NL_LANGINFO)
  mem2str( fmt, nl_langinfo(D_T_FMT), DIM(fmt)-3 );
  if (!strstr( fmt, "%Z" ))
    strcat( fmt, " %Z");
  /* NOTE: gcc -Wformat-noliteral will complain here.  I have found no
     way to suppress this warning.  */
  strftime (buffer, DIM(buffer)-1, fmt, tp);
# else
#  if HAVE_W32_SYSTEM
  {
    static int done;

    if (!done)
      {
        /* The locale names as used by Windows are in the form
         * "German_Germany.1252" or "German_Austria.1252" with
         * alternate names similar to Unix, e.g. "de-DE".  However
         * that is the theory.  On current Windows and Mingw the
         * alternate names do not work.  We would need a table to map
         * them from the short names as provided by gpgrt to the long
         * names and append some code page.  For now we use "" and
         * take the locale from the user's system settings.  Thus the
         * standard Unix envvars don't work for time and may mismatch
         * with the string translations.  The new UCRT available since
         * 2018 has a lot of additional support but that will for sure
         * break other things.  We should move to ISO strings to get
         * rid of such problems.  */
        setlocale (LC_TIME, ".UTF8");
        done = 1;
        /* log_debug ("LC_ALL  now '%s'\n", setlocale (LC_ALL, NULL)); */
        /* log_debug ("LC_TIME now '%s'\n", setlocale (LC_TIME, NULL)); */
      }
  }
#  endif
   /* FIXME: we should check whether the locale appends a " %Z" These
    * locales from glibc don't put the " %Z": fi_FI hr_HR ja_JP lt_LT
    * lv_LV POSIX ru_RU ru_SU sv_FI sv_SE zh_CN.  */
  strftime (buffer, DIM(buffer)-1, "%c %Z", tp);
# endif
  buffer[DIM(buffer)-1] = 0;
#else
  mem2str( buffer, asctime(tp), DIM(buffer) );
#endif
  return buffer;
}


/* Return the timestamp STAMP in RFC-2822 format.  This is always done
 * in the C locale.  We return the gmtime to avoid computing the
 * timezone. The caller must release the returned string.
 *
 * Example: "Mon, 27 Jun 2016 1:42:00 +0000".
 */
char *
rfctimestamp (u32 stamp)
{
  time_t atime = stamp;
  struct tm tmbuf, *tp;


  if (IS_INVALID_TIME_T (atime))
    {
      gpg_err_set_errno (EINVAL);
      return NULL;
    }

  tp = gnupg_gmtime (&atime, &tmbuf);
  if (!tp)
    return NULL;
  return xtryasprintf ("%.3s, %02d %.3s %04d %02d:%02d:%02d +0000",
                       &"SunMonTueWedThuFriSat"[(tp->tm_wday%7)*3],
                       tp->tm_mday,
                       &"JanFebMarAprMayJunJulAugSepOctNovDec"
                       [(tp->tm_mon%12)*3],
                       tp->tm_year + 1900,
                       tp->tm_hour,
                       tp->tm_min,
                       tp->tm_sec);
}


static int
days_per_year (int y)
{
  int s ;

  s = !(y % 4);
  if ( !(y % 100))
    if ((y%400))
      s = 0;
  return s ? 366 : 365;
}

static int
days_per_month (int y, int m)
{
  int s;

  switch(m)
    {
    case 1: case 3: case 5: case 7: case 8: case 10: case 12:
      return 31 ;
    case 2:
      s = !(y % 4);
      if (!(y % 100))
        if ((y % 400))
          s = 0;
      return s? 29 : 28 ;
    case 4: case 6: case 9: case 11:
      return 30;
    }
  BUG();
}


/* Convert YEAR, MONTH and DAY into the Julian date.  We assume that
   it is already noon.  We do not support dates before 1582-10-15. */
static unsigned long
date2jd (int year, int month, int day)
{
  unsigned long jd;

  jd = 365L * year + 31 * (month-1) + day + JD_DIFF;
  if (month < 3)
    year-- ;
  else
    jd -= (4 * month + 23) / 10;

  jd += year / 4 - ((year / 100 + 1) *3) / 4;

  return jd ;
}

/* Convert a Julian date back to YEAR, MONTH and DAY.  Return day of
   the year or 0 on error.  This function uses some more or less
   arbitrary limits, most important is that days before 1582 are not
   supported. */
static int
jd2date (unsigned long jd, int *year, int *month, int *day)
{
  int y, m, d;
  long delta;

  if (!jd)
    return 0 ;
  if (jd < 1721425 || jd > 2843085)
    return 0;

  y = (jd - JD_DIFF) / 366;
  d = m = 1;

  while ((delta = jd - date2jd (y, m, d)) > days_per_year (y))
    y++;

  m = (delta / 31) + 1;
  while( (delta = jd - date2jd (y, m, d)) > days_per_month (y,m))
    if (++m > 12)
      {
        m = 1;
        y++;
      }

  d = delta + 1 ;
  if (d > days_per_month (y, m))
    {
      d = 1;
      m++;
    }
  if (m > 12)
    {
      m = 1;
      y++;
    }

  if (year)
    *year = y;
  if (month)
    *month = m;
  if (day)
    *day = d ;

  return (jd - date2jd (y, 1, 1)) + 1;
}


/* Check that the 15 bytes in ATIME represent a valid ISO time.  Note
   that this function does not expect a string but a plain 15 byte
   isotime buffer. */
gpg_error_t
check_isotime (const gnupg_isotime_t atime)
{
  int i;
  const char *s;

  if (!*atime)
    return gpg_error (GPG_ERR_NO_VALUE);

  for (s=atime, i=0; i < 8; i++, s++)
    if (!digitp (s))
      return gpg_error (GPG_ERR_INV_TIME);
  if (*s != 'T')
      return gpg_error (GPG_ERR_INV_TIME);
  for (s++, i=9; i < 15; i++, s++)
    if (!digitp (s))
      return gpg_error (GPG_ERR_INV_TIME);
  return 0;
}


/* Dump the ISO time T to the log stream without a LF.  */
void
dump_isotime (const gnupg_isotime_t t)
{
  if (!t || !*t)
    log_printf ("%s", _("[none]"));
  else
    log_printf ("%.4s-%.2s-%.2s %.2s:%.2s:%s",
                t, t+4, t+6, t+9, t+11, t+13);
}


/* Copy one ISO date to another, this is inline so that we can do a
   minimal sanity check.  A null date (empty string) is allowed.  */
void
gnupg_copy_time (gnupg_isotime_t d, const gnupg_isotime_t s)
{
  if (*s)
    {
      if ((strlen (s) != 15 || s[8] != 'T'))
        BUG();
      memcpy (d, s, 15);
      d[15] = 0;
    }
  else
    *d = 0;
}


/* Add SECONDS to ATIME.  SECONDS may not be negative and is limited
   to about the equivalent of 62 years which should be more then
   enough for our purposes. */
gpg_error_t
add_seconds_to_isotime (gnupg_isotime_t atime, int nseconds)
{
  gpg_error_t err;
  int year, month, day, hour, minute, sec, ndays;
  unsigned long jd;

  err = check_isotime (atime);
  if (err)
    return err;

  if (nseconds < 0 || nseconds >= (0x7fffffff - 61) )
    return gpg_error (GPG_ERR_INV_VALUE);

  year  = atoi_4 (atime+0);
  month = atoi_2 (atime+4);
  day   = atoi_2 (atime+6);
  hour  = atoi_2 (atime+9);
  minute= atoi_2 (atime+11);
  sec   = atoi_2 (atime+13);

  if (year <= 1582) /* The julian date functions don't support this. */
    return gpg_error (GPG_ERR_INV_VALUE);

  sec    += nseconds;
  minute += sec/60;
  sec    %= 60;
  hour   += minute/60;
  minute %= 60;
  ndays  = hour/24;
  hour   %= 24;

  jd = date2jd (year, month, day) + ndays;
  jd2date (jd, &year, &month, &day);

  if (year > 9999 || month > 12 || day > 31
      || year < 0 || month < 1 || day < 1)
    return gpg_error (GPG_ERR_INV_VALUE);

  snprintf (atime, 16, "%04d%02d%02dT%02d%02d%02d",
            year, month, day, hour, minute, sec);
  return 0;
}


gpg_error_t
add_days_to_isotime (gnupg_isotime_t atime, int ndays)
{
  gpg_error_t err;
  int year, month, day, hour, minute, sec;
  unsigned long jd;

  err = check_isotime (atime);
  if (err)
    return err;

  if (ndays < 0 || ndays >= 9999*366 )
    return gpg_error (GPG_ERR_INV_VALUE);

  year  = atoi_4 (atime+0);
  month = atoi_2 (atime+4);
  day   = atoi_2 (atime+6);
  hour  = atoi_2 (atime+9);
  minute= atoi_2 (atime+11);
  sec   = atoi_2 (atime+13);

  if (year <= 1582) /* The julian date functions don't support this. */
    return gpg_error (GPG_ERR_INV_VALUE);

  jd = date2jd (year, month, day) + ndays;
  jd2date (jd, &year, &month, &day);

  if (year > 9999 || month > 12 || day > 31
      || year < 0 || month < 1 || day < 1)
    return gpg_error (GPG_ERR_INV_VALUE);

  snprintf (atime, 16, "%04d%02d%02dT%02d%02d%02d",
            year, month, day, hour, minute, sec);
  return 0;
}
