text: /* Written by Richard Levitte (richard@levitte.org) for the OpenSSL
   * project 2001.
   * Written by Dr Stephen N Henson (steve@openssl.org) for the OpenSSL
   * project 2008.
   */
  /* ====================================================================
   * Copyright (c) 2001 The OpenSSL Project.  All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions
   * are met:
   *
   * 1. Redistributions of source code must retain the above copyright
   *    notice, this list of conditions and the following disclaimer.
   *
   * 2. Redistributions in binary form must reproduce the above copyright
   *    notice, this list of conditions and the following disclaimer in
   *    the documentation and/or other materials provided with the
   *    distribution.
   *
   * 3. All advertising materials mentioning features or use of this
   *    software must display the following acknowledgment:
   *    "This product includes software developed by the OpenSSL Project
   *    for use in the OpenSSL Toolkit. (http://www.OpenSSL.org/)"
   *
   * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
   *    endorse or promote products derived from this software without
   *    prior written permission. For written permission, please contact
   *    licensing@OpenSSL.org.
   *
   * 5. Products derived from this software may not be called "OpenSSL"
   *    nor may "OpenSSL" appear in their names without prior written
   *    permission of the OpenSSL Project.
   *
   * 6. Redistributions of any form whatsoever must retain the following
   *    acknowledgment:
   *    "This product includes software developed by the OpenSSL Project
   *    for use in the OpenSSL Toolkit (http://www.OpenSSL.org/)"
   *
   * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
   * EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
   * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
   * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
   * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
   * OF THE POSSIBILITY OF SUCH DAMAGE.
   * ====================================================================
   *
   * This product includes cryptographic software written by Eric Young
   * (eay@cryptsoft.com).  This product includes software written by Tim
   * Hudson (tjh@cryptsoft.com). */
  
  #if !defined(_POSIX_C_SOURCE)
  #define _POSIX_C_SOURCE 201410L  /* for gmtime_r */
  #endif
  
  #include <openssl/time_support.h>
  
  #define SECS_PER_DAY (24 * 60 * 60)
  
  struct tm *OPENSSL_gmtime(const time_t *time, struct tm *result) {
  #if defined(OPENSSL_WINDOWS)
    if (gmtime_s(result, time)) {
      return NULL;
    }
    return result;
  #else
    return gmtime_r(time, result);
  #endif
  }
  
  /* Convert date to and from julian day Uses Fliegel & Van Flandern algorithm */
  static long date_to_julian(int y, int m, int d) {
    return (1461 * (y + 4800 + (m - 14) / 12)) / 4 +
           (367 * (m - 2 - 12 * ((m - 14) / 12))) / 12 -
           (3 * ((y + 4900 + (m - 14) / 12) / 100)) / 4 + d - 32075;
  }
  
  static void julian_to_date(long jd, int *y, int *m, int *d) {
    long L = jd + 68569;
    long n = (4 * L) / 146097;
    long i, j;
  
    L = L - (146097 * n + 3) / 4;
    i = (4000 * (L + 1)) / 1461001;
    L = L - (1461 * i) / 4 + 31;
    j = (80 * L) / 2447;
    *d = L - (2447 * j) / 80;
    L = j / 11;
    *m = j + 2 - (12 * L);
    *y = 100 * (n - 49) + i + L;
  }
  
  /* Convert tm structure and offset into julian day and seconds */
  static int julian_adj(const struct tm *tm, int off_day, long offset_sec,
                        long *pday, int *psec) {
    int offset_hms, offset_day;
    long time_jd;
    int time_year, time_month, time_day;
    /* split offset into days and day seconds */
    offset_day = offset_sec / SECS_PER_DAY;
    /* Avoid sign issues with % operator */
    offset_hms = offset_sec - (offset_day * SECS_PER_DAY);
    offset_day += off_day;
    /* Add current time seconds to offset */
    offset_hms += tm->tm_hour * 3600 + tm->tm_min * 60 + tm->tm_sec;
    /* Adjust day seconds if overflow */
    if (offset_hms >= SECS_PER_DAY) {
      offset_day++;
      offset_hms -= SECS_PER_DAY;
    } else if (offset_hms < 0) {
      offset_day--;
      offset_hms += SECS_PER_DAY;
    }
  
    /* Convert date of time structure into a Julian day number. */
  
    time_year = tm->tm_year + 1900;
    time_month = tm->tm_mon + 1;
    time_day = tm->tm_mday;
  
    time_jd = date_to_julian(time_year, time_month, time_day);
  
    /* Work out Julian day of new date */
    time_jd += offset_day;
  
    if (time_jd < 0) {
      return 0;
    }
  
    *pday = time_jd;
    *psec = offset_hms;
    return 1;
  }
  
  int OPENSSL_gmtime_adj(struct tm *tm, int off_day, long offset_sec) {
    int time_sec, time_year, time_month, time_day;
    long time_jd;
  
    /* Convert time and offset into julian day and seconds */
    if (!julian_adj(tm, off_day, offset_sec, &time_jd, &time_sec)) {
      return 0;
    }
  
    /* Convert Julian day back to date */
  
    julian_to_date(time_jd, &time_year, &time_month, &time_day);
  
    if (time_year < 1900 || time_year > 9999) {
      return 0;
    }
  
    /* Update tm structure */
  
    tm->tm_year = time_year - 1900;
    tm->tm_mon = time_month - 1;
    tm->tm_mday = time_day;
  
    tm->tm_hour = time_sec / 3600;
    tm->tm_min = (time_sec / 60) % 60;
    tm->tm_sec = time_sec % 60;
  
    return 1;
  }
  
  int OPENSSL_gmtime_diff(int *pday, int *psec, const struct tm *from,
                          const struct tm *to) {
    int from_sec, to_sec, diff_sec;
    long from_jd, to_jd, diff_day;
  
    if (!julian_adj(from, 0, 0, &from_jd, &from_sec)) {
      return 0;
    }
    if (!julian_adj(to, 0, 0, &to_jd, &to_sec)) {
      return 0;
    }
  
    diff_day = to_jd - from_jd;
    diff_sec = to_sec - from_sec;
    /* Adjust differences so both positive or both negative */
    if (diff_day > 0 && diff_sec < 0) {
      diff_day--;
      diff_sec += SECS_PER_DAY;
    }
    if (diff_day < 0 && diff_sec > 0) {
      diff_day++;
      diff_sec -= SECS_PER_DAY;
    }
  
    if (pday) {
      *pday = (int)diff_day;
    }
    if (psec) {
      *psec = diff_sec;
    }
  
    return 1;
  }
  