third_party/WebKit/Source/wtf/DateMath.cpp

/*
 * Copyright (C) 1999-2000 Harri Porten (porten@kde.org)
 * Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
 * Copyright (C) 2009 Google Inc. All rights reserved.
 * Copyright (C) 2007-2009 Torch Mobile, Inc.
 * Copyright (C) 2010 &yet, LLC. (nate@andyet.net)
 *
 * The Original Code is Mozilla Communicator client code, released
 * March 31, 1998.
 *
 * The Initial Developer of the Original Code is
 * Netscape Communications Corporation.
 * Portions created by the Initial Developer are Copyright (C) 1998
 * the Initial Developer. All Rights Reserved.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 USA
 *
 * Alternatively, the contents of this file may be used under the terms
 * of either the Mozilla Public License Version 1.1, found at
 * http://www.mozilla.org/MPL/ (the "MPL") or the GNU General Public
 * License Version 2.0, found at http://www.fsf.org/copyleft/gpl.html
 * (the "GPL"), in which case the provisions of the MPL or the GPL are
 * applicable instead of those above.  If you wish to allow use of your
 * version of this file only under the terms of one of those two
 * licenses (the MPL or the GPL) and not to allow others to use your
 * version of this file under the LGPL, indicate your decision by
 * deletingthe provisions above and replace them with the notice and
 * other provisions required by the MPL or the GPL, as the case may be.
 * If you do not delete the provisions above, a recipient may use your
 * version of this file under any of the LGPL, the MPL or the GPL.

 * Copyright 2006-2008 the V8 project authors. All rights reserved.
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *     * 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.
 *     * Neither the name of Google Inc. nor the names of its
 *       contributors may be used to endorse or promote products derived
 *       from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS 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 COPYRIGHT
 * OWNER OR 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.
 */

#include "wtf/DateMath.h"

#include "wtf/ASCIICType.h"
#include "wtf/Assertions.h"
#include "wtf/CurrentTime.h"
#include "wtf/MathExtras.h"
#include "wtf/StdLibExtras.h"
#include "wtf/StringExtras.h"
#include "wtf/text/StringBuilder.h"
#include <algorithm>
#include <limits.h>
#include <limits>
#include <math.h>
#include <stdlib.h>
#include <time.h>

#if OS(WIN)
#include <windows.h>
#else
#include <sys/time.h>
#endif

namespace WTF {

/* Constants */

static const double hoursPerDay = 24.0;
static const double secondsPerDay = 24.0 * 60.0 * 60.0;

static const double maxUnixTime = 2145859200.0;  // 12/31/2037
static const double kMinimumECMADateInMs = -8640000000000000.0;
static const double kMaximumECMADateInMs = 8640000000000000.0;

// Day of year for the first day of each month, where index 0 is January, and
// day 0 is January 1.  First for non-leap years, then for leap years.
static const int firstDayOfMonth[2][12] = {
    {0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334},
    {0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335}};

static inline void getLocalTime(const time_t* localTime, struct tm* localTM) {
#if COMPILER(MSVC)
  localtime_s(localTM, localTime);
#else
  localtime_r(localTime, localTM);
#endif
}

bool isLeapYear(int year) {
  if (year % 4 != 0)
    return false;
  if (year % 400 == 0)
    return true;
  if (year % 100 == 0)
    return false;
  return true;
}

static inline int daysInYear(int year) {
  return 365 + isLeapYear(year);
}

static inline double daysFrom1970ToYear(int year) {
  // The Gregorian Calendar rules for leap years:
  // Every fourth year is a leap year.  2004, 2008, and 2012 are leap years.
  // However, every hundredth year is not a leap year.  1900 and 2100 are not
  // leap years.
  // Every four hundred years, there's a leap year after all.  2000 and 2400 are
  // leap years.

  static const int leapDaysBefore1971By4Rule = 1970 / 4;
  static const int excludedLeapDaysBefore1971By100Rule = 1970 / 100;
  static const int leapDaysBefore1971By400Rule = 1970 / 400;

  const double yearMinusOne = year - 1;
  const double yearsToAddBy4Rule =
      floor(yearMinusOne / 4.0) - leapDaysBefore1971By4Rule;
  const double yearsToExcludeBy100Rule =
      floor(yearMinusOne / 100.0) - excludedLeapDaysBefore1971By100Rule;
  const double yearsToAddBy400Rule =
      floor(yearMinusOne / 400.0) - leapDaysBefore1971By400Rule;

  return 365.0 * (year - 1970) + yearsToAddBy4Rule - yearsToExcludeBy100Rule +
         yearsToAddBy400Rule;
}

static double msToDays(double ms) {
  return floor(ms / msPerDay);
}

static void appendTwoDigitNumber(StringBuilder& builder, int number) {
  ASSERT(number >= 0 && number < 100);
  if (number <= 9)
    builder.append('0');
  builder.appendNumber(number);
}

int msToYear(double ms) {
  DCHECK(std::isfinite(ms));
  DCHECK_GE(ms, kMinimumECMADateInMs);
  DCHECK_LE(ms, kMaximumECMADateInMs);
  int approxYear = static_cast<int>(floor(ms / (msPerDay * 365.2425)) + 1970);
  double msFromApproxYearTo1970 = msPerDay * daysFrom1970ToYear(approxYear);
  if (msFromApproxYearTo1970 > ms)
    return approxYear - 1;
  if (msFromApproxYearTo1970 + msPerDay * daysInYear(approxYear) <= ms)
    return approxYear + 1;
  return approxYear;
}

int dayInYear(double ms, int year) {
  return static_cast<int>(msToDays(ms) - daysFrom1970ToYear(year));
}

static inline double msToMilliseconds(double ms) {
  double result = fmod(ms, msPerDay);
  if (result < 0)
    result += msPerDay;
  return result;
}

int monthFromDayInYear(int dayInYear, bool leapYear) {
  const int d = dayInYear;
  int step;

  if (d < (step = 31))
    return 0;
  step += (leapYear ? 29 : 28);
  if (d < step)
    return 1;
  if (d < (step += 31))
    return 2;
  if (d < (step += 30))
    return 3;
  if (d < (step += 31))
    return 4;
  if (d < (step += 30))
    return 5;
  if (d < (step += 31))
    return 6;
  if (d < (step += 31))
    return 7;
  if (d < (step += 30))
    return 8;
  if (d < (step += 31))
    return 9;
  if (d < (step += 30))
    return 10;
  return 11;
}

static inline bool checkMonth(int dayInYear,
                              int& startDayOfThisMonth,
                              int& startDayOfNextMonth,
                              int daysInThisMonth) {
  startDayOfThisMonth = startDayOfNextMonth;
  startDayOfNextMonth += daysInThisMonth;
  return (dayInYear <= startDayOfNextMonth);
}

int dayInMonthFromDayInYear(int dayInYear, bool leapYear) {
  const int d = dayInYear;
  int step;
  int next = 30;

  if (d <= next)
    return d + 1;
  const int daysInFeb = (leapYear ? 29 : 28);
  if (checkMonth(d, step, next, daysInFeb))
    return d - step;
  if (checkMonth(d, step, next, 31))
    return d - step;
  if (checkMonth(d, step, next, 30))
    return d - step;
  if (checkMonth(d, step, next, 31))
    return d - step;
  if (checkMonth(d, step, next, 30))
    return d - step;
  if (checkMonth(d, step, next, 31))
    return d - step;
  if (checkMonth(d, step, next, 31))
    return d - step;
  if (checkMonth(d, step, next, 30))
    return d - step;
  if (checkMonth(d, step, next, 31))
    return d - step;
  if (checkMonth(d, step, next, 30))
    return d - step;
  step = next;
  return d - step;
}

int dayInYear(int year, int month, int day) {
  return firstDayOfMonth[isLeapYear(year)][month] + day - 1;
}

double dateToDaysFrom1970(int year, int month, int day) {
  year += month / 12;

  month %= 12;
  if (month < 0) {
    month += 12;
    --year;
  }

  double yearday = floor(daysFrom1970ToYear(year));
  ASSERT((year >= 1970 && yearday >= 0) || (year < 1970 && yearday < 0));
  return yearday + dayInYear(year, month, day);
}

// There is a hard limit at 2038 that we currently do not have a workaround
// for (rdar://problem/5052975).
static inline int maximumYearForDST() {
  return 2037;
}

static inline double jsCurrentTime() {
  // JavaScript doesn't recognize fractions of a millisecond.
  return floor(WTF::currentTimeMS());
}

static inline int minimumYearForDST() {
  // Because of the 2038 issue (see maximumYearForDST) if the current year is
  // greater than the max year minus 27 (2010), we want to use the max year
  // minus 27 instead, to ensure there is a range of 28 years that all years
  // can map to.
  return std::min(msToYear(jsCurrentTime()), maximumYearForDST() - 27);
}

// Find an equivalent year for the one given, where equivalence is deterined by
// the two years having the same leapness and the first day of the year, falling
// on the same day of the week.
//
// This function returns a year between this current year and 2037, however this
// function will potentially return incorrect results if the current year is
// after 2010, (rdar://problem/5052975), if the year passed in is before 1900
// or after 2100, (rdar://problem/5055038).
static int equivalentYearForDST(int year) {
  // It is ok if the cached year is not the current year as long as the rules
  // for DST did not change between the two years; if they did the app would
  // need to be restarted.
  static int minYear = minimumYearForDST();
  int maxYear = maximumYearForDST();

  int difference;
  if (year > maxYear)
    difference = minYear - year;
  else if (year < minYear)
    difference = maxYear - year;
  else
    return year;

  int quotient = difference / 28;
  int product = (quotient)*28;

  year += product;
  ASSERT((year >= minYear && year <= maxYear) ||
         (product - year ==
          static_cast<int>(std::numeric_limits<double>::quiet_NaN())));
  return year;
}

static double calculateUTCOffset() {
#if OS(WIN)
  TIME_ZONE_INFORMATION timeZoneInformation;
  GetTimeZoneInformation(&timeZoneInformation);
  int32_t bias = timeZoneInformation.Bias + timeZoneInformation.StandardBias;
  return -bias * 60 * 1000;
#else
  time_t localTime = time(0);
  tm localt;
  getLocalTime(&localTime, &localt);

  // tm_gmtoff includes any daylight savings offset, so subtract it.
  return static_cast<double>(localt.tm_gmtoff * msPerSecond -
                             (localt.tm_isdst > 0 ? msPerHour : 0));
#endif
}

/*
 * Get the DST offset for the time passed in.
 */
static double calculateDSTOffsetSimple(double localTimeSeconds,
                                       double utcOffset) {
  if (localTimeSeconds > maxUnixTime)
    localTimeSeconds = maxUnixTime;
  else if (localTimeSeconds <
           0)  // Go ahead a day to make localtime work (does not work with 0)
    localTimeSeconds += secondsPerDay;

  // FIXME: time_t has a potential problem in 2038
  time_t localTime = static_cast<time_t>(localTimeSeconds);

  tm localTM;
  getLocalTime(&localTime, &localTM);

  return localTM.tm_isdst > 0 ? msPerHour : 0;
}

// Get the DST offset, given a time in UTC
static double calculateDSTOffset(double ms, double utcOffset) {
  // On macOS, the call to localtime (see calculateDSTOffsetSimple) will return
  // historically accurate DST information (e.g. New Zealand did not have DST
  // from 1946 to 1974) however the JavaScript standard explicitly dictates
  // that historical information should not be considered when determining DST.
  // For this reason we shift away from years that localtime can handle but
  // would return historically accurate information.
  int year = msToYear(ms);
  int equivalentYear = equivalentYearForDST(year);
  if (year != equivalentYear) {
    bool leapYear = isLeapYear(year);
    int dayInYearLocal = dayInYear(ms, year);
    int dayInMonth = dayInMonthFromDayInYear(dayInYearLocal, leapYear);
    int month = monthFromDayInYear(dayInYearLocal, leapYear);
    double day = dateToDaysFrom1970(equivalentYear, month, dayInMonth);
    ms = (day * msPerDay) + msToMilliseconds(ms);
  }

  return calculateDSTOffsetSimple(ms / msPerSecond, utcOffset);
}

void initializeDates() {
#if ENABLE(ASSERT)
  static bool alreadyInitialized;
  ASSERT(!alreadyInitialized);
  alreadyInitialized = true;
#endif

  equivalentYearForDST(
      2000);  // Need to call once to initialize a static used in this function.
}

static inline double ymdhmsToSeconds(int year,
                                     long mon,
                                     long day,
                                     long hour,
                                     long minute,
                                     double second) {
  double days =
      (day - 32075) + floor(1461 * (year + 4800.0 + (mon - 14) / 12) / 4) +
      367 * (mon - 2 - (mon - 14) / 12 * 12) / 12 -
      floor(3 * ((year + 4900.0 + (mon - 14) / 12) / 100) / 4) - 2440588;
  return ((days * hoursPerDay + hour) * minutesPerHour + minute) *
             secondsPerMinute +
         second;
}

// We follow the recommendation of RFC 2822 to consider all
// obsolete time zones not listed here equivalent to "-0000".
static const struct KnownZone {
#if !OS(WIN)
  const
#endif
      char tzName[4];
  int tzOffset;
} known_zones[] = {{"UT", 0},     {"GMT", 0},    {"EST", -300}, {"EDT", -240},
                   {"CST", -360}, {"CDT", -300}, {"MST", -420}, {"MDT", -360},
                   {"PST", -480}, {"PDT", -420}};

inline static void skipSpacesAndComments(const char*& s) {
  int nesting = 0;
  char ch;
  while ((ch = *s)) {
    if (!isASCIISpace(ch)) {
      if (ch == '(')
        nesting++;
      else if (ch == ')' && nesting > 0)
        nesting--;
      else if (nesting == 0)
        break;
    }
    s++;
  }
}

// returns 0-11 (Jan-Dec); -1 on failure
static int findMonth(const char* monthStr) {
  ASSERT(monthStr);
  char needle[4];
  for (int i = 0; i < 3; ++i) {
    if (!*monthStr)
      return -1;
    needle[i] = static_cast<char>(toASCIILower(*monthStr++));
  }
  needle[3] = '\0';
  const char* haystack = "janfebmaraprmayjunjulaugsepoctnovdec";
  const char* str = strstr(haystack, needle);
  if (str) {
    int position = static_cast<int>(str - haystack);
    if (position % 3 == 0)
      return position / 3;
  }
  return -1;
}

static bool parseInt(const char* string,
                     char** stopPosition,
                     int base,
                     int* result) {
  long longResult = strtol(string, stopPosition, base);
  // Avoid the use of errno as it is not available on Windows CE
  if (string == *stopPosition ||
      longResult <= std::numeric_limits<int>::min() ||
      longResult >= std::numeric_limits<int>::max())
    return false;
  *result = static_cast<int>(longResult);
  return true;
}

static bool parseLong(const char* string,
                      char** stopPosition,
                      int base,
                      long* result) {
  *result = strtol(string, stopPosition, base);
  // Avoid the use of errno as it is not available on Windows CE
  if (string == *stopPosition || *result == std::numeric_limits<long>::min() ||
      *result == std::numeric_limits<long>::max())
    return false;
  return true;
}

// Odd case where 'exec' is allowed to be 0, to accomodate a caller in WebCore.
static double parseDateFromNullTerminatedCharacters(const char* dateString,
                                                    bool& haveTZ,
                                                    int& offset) {
  haveTZ = false;
  offset = 0;

  // This parses a date in the form:
  //     Tuesday, 09-Nov-99 23:12:40 GMT
  // or
  //     Sat, 01-Jan-2000 08:00:00 GMT
  // or
  //     Sat, 01 Jan 2000 08:00:00 GMT
  // or
  //     01 Jan 99 22:00 +0100    (exceptions in rfc822/rfc2822)
  // ### non RFC formats, added for Javascript:
  //     [Wednesday] January 09 1999 23:12:40 GMT
  //     [Wednesday] January 09 23:12:40 GMT 1999
  //
  // We ignore the weekday.

  // Skip leading space
  skipSpacesAndComments(dateString);

  long month = -1;
  const char* wordStart = dateString;
  // Check contents of first words if not number
  while (*dateString && !isASCIIDigit(*dateString)) {
    if (isASCIISpace(*dateString) || *dateString == '(') {
      if (dateString - wordStart >= 3)
        month = findMonth(wordStart);
      skipSpacesAndComments(dateString);
      wordStart = dateString;
    } else {
      dateString++;
    }
  }

  // Missing delimiter between month and day (like "January29")?
  if (month == -1 && wordStart != dateString)
    month = findMonth(wordStart);

  skipSpacesAndComments(dateString);

  if (!*dateString)
    return std::numeric_limits<double>::quiet_NaN();

  // ' 09-Nov-99 23:12:40 GMT'
  char* newPosStr;
  long day;
  if (!parseLong(dateString, &newPosStr, 10, &day))
    return std::numeric_limits<double>::quiet_NaN();
  dateString = newPosStr;

  if (!*dateString)
    return std::numeric_limits<double>::quiet_NaN();

  if (day < 0)
    return std::numeric_limits<double>::quiet_NaN();

  int year = 0;
  if (day > 31) {
    // ### where is the boundary and what happens below?
    if (*dateString != '/')
      return std::numeric_limits<double>::quiet_NaN();
    // looks like a YYYY/MM/DD date
    if (!*++dateString)
      return std::numeric_limits<double>::quiet_NaN();
    if (day <= std::numeric_limits<int>::min() ||
        day >= std::numeric_limits<int>::max())
      return std::numeric_limits<double>::quiet_NaN();
    year = static_cast<int>(day);
    if (!parseLong(dateString, &newPosStr, 10, &month))
      return std::numeric_limits<double>::quiet_NaN();
    month -= 1;
    dateString = newPosStr;
    if (*dateString++ != '/' || !*dateString)
      return std::numeric_limits<double>::quiet_NaN();
    if (!parseLong(dateString, &newPosStr, 10, &day))
      return std::numeric_limits<double>::quiet_NaN();
    dateString = newPosStr;
  } else if (*dateString == '/' && month == -1) {
    dateString++;
    // This looks like a MM/DD/YYYY date, not an RFC date.
    month = day - 1;  // 0-based
    if (!parseLong(dateString, &newPosStr, 10, &day))
      return std::numeric_limits<double>::quiet_NaN();
    if (day < 1 || day > 31)
      return std::numeric_limits<double>::quiet_NaN();
    dateString = newPosStr;
    if (*dateString == '/')
      dateString++;
    if (!*dateString)
      return std::numeric_limits<double>::quiet_NaN();
  } else {
    if (*dateString == '-')
      dateString++;

    skipSpacesAndComments(dateString);

    if (*dateString == ',')
      dateString++;

    if (month == -1) {  // not found yet
      month = findMonth(dateString);
      if (month == -1)
        return std::numeric_limits<double>::quiet_NaN();

      while (*dateString && *dateString != '-' && *dateString != ',' &&
             !isASCIISpace(*dateString))
        dateString++;

      if (!*dateString)
        return std::numeric_limits<double>::quiet_NaN();

      // '-99 23:12:40 GMT'
      if (*dateString != '-' && *dateString != '/' && *dateString != ',' &&
          !isASCIISpace(*dateString))
        return std::numeric_limits<double>::quiet_NaN();
      dateString++;
    }
  }

  if (month < 0 || month > 11)
    return std::numeric_limits<double>::quiet_NaN();

  // '99 23:12:40 GMT'
  if (year <= 0 && *dateString) {
    if (!parseInt(dateString, &newPosStr, 10, &year))
      return std::numeric_limits<double>::quiet_NaN();
  }

  // Don't fail if the time is missing.
  long hour = 0;
  long minute = 0;
  long second = 0;
  if (!*newPosStr) {
    dateString = newPosStr;
  } else {
    // ' 23:12:40 GMT'
    if (!(isASCIISpace(*newPosStr) || *newPosStr == ',')) {
      if (*newPosStr != ':')
        return std::numeric_limits<double>::quiet_NaN();
      // There was no year; the number was the hour.
      year = -1;
    } else {
      // in the normal case (we parsed the year), advance to the next number
      dateString = ++newPosStr;
      skipSpacesAndComments(dateString);
    }

    parseLong(dateString, &newPosStr, 10, &hour);
    // Do not check for errno here since we want to continue
    // even if errno was set becasue we are still looking
    // for the timezone!

    // Read a number? If not, this might be a timezone name.
    if (newPosStr != dateString) {
      dateString = newPosStr;

      if (hour < 0 || hour > 23)
        return std::numeric_limits<double>::quiet_NaN();

      if (!*dateString)
        return std::numeric_limits<double>::quiet_NaN();

      // ':12:40 GMT'
      if (*dateString++ != ':')
        return std::numeric_limits<double>::quiet_NaN();

      if (!parseLong(dateString, &newPosStr, 10, &minute))
        return std::numeric_limits<double>::quiet_NaN();
      dateString = newPosStr;

      if (minute < 0 || minute > 59)
        return std::numeric_limits<double>::quiet_NaN();

      // ':40 GMT'
      if (*dateString && *dateString != ':' && !isASCIISpace(*dateString))
        return std::numeric_limits<double>::quiet_NaN();

      // seconds are optional in rfc822 + rfc2822
      if (*dateString == ':') {
        dateString++;

        if (!parseLong(dateString, &newPosStr, 10, &second))
          return std::numeric_limits<double>::quiet_NaN();
        dateString = newPosStr;

        if (second < 0 || second > 59)
          return std::numeric_limits<double>::quiet_NaN();
      }

      skipSpacesAndComments(dateString);

      if (strncasecmp(dateString, "AM", 2) == 0) {
        if (hour > 12)
          return std::numeric_limits<double>::quiet_NaN();
        if (hour == 12)
          hour = 0;
        dateString += 2;
        skipSpacesAndComments(dateString);
      } else if (strncasecmp(dateString, "PM", 2) == 0) {
        if (hour > 12)
          return std::numeric_limits<double>::quiet_NaN();
        if (hour != 12)
          hour += 12;
        dateString += 2;
        skipSpacesAndComments(dateString);
      }
    }
  }

  // The year may be after the time but before the time zone.
  if (isASCIIDigit(*dateString) && year == -1) {
    if (!parseInt(dateString, &newPosStr, 10, &year))
      return std::numeric_limits<double>::quiet_NaN();
    dateString = newPosStr;
    skipSpacesAndComments(dateString);
  }

  // Don't fail if the time zone is missing.
  // Some websites omit the time zone (4275206).
  if (*dateString) {
    if (strncasecmp(dateString, "GMT", 3) == 0 ||
        strncasecmp(dateString, "UTC", 3) == 0) {
      dateString += 3;
      haveTZ = true;
    }

    if (*dateString == '+' || *dateString == '-') {
      int o;
      if (!parseInt(dateString, &newPosStr, 10, &o))
        return std::numeric_limits<double>::quiet_NaN();
      dateString = newPosStr;

      if (o < -9959 || o > 9959)
        return std::numeric_limits<double>::quiet_NaN();

      int sgn = (o < 0) ? -1 : 1;
      o = abs(o);
      if (*dateString != ':') {
        if (o >= 24)
          offset = ((o / 100) * 60 + (o % 100)) * sgn;
        else
          offset = o * 60 * sgn;
      } else {         // GMT+05:00
        ++dateString;  // skip the ':'
        int o2;
        if (!parseInt(dateString, &newPosStr, 10, &o2))
          return std::numeric_limits<double>::quiet_NaN();
        dateString = newPosStr;
        offset = (o * 60 + o2) * sgn;
      }
      haveTZ = true;
    } else {
      for (size_t i = 0; i < WTF_ARRAY_LENGTH(known_zones); ++i) {
        if (0 == strncasecmp(dateString, known_zones[i].tzName,
                             strlen(known_zones[i].tzName))) {
          offset = known_zones[i].tzOffset;
          dateString += strlen(known_zones[i].tzName);
          haveTZ = true;
          break;
        }
      }
    }
  }

  skipSpacesAndComments(dateString);

  if (*dateString && year == -1) {
    if (!parseInt(dateString, &newPosStr, 10, &year))
      return std::numeric_limits<double>::quiet_NaN();
    dateString = newPosStr;
    skipSpacesAndComments(dateString);
  }

  // Trailing garbage
  if (*dateString)
    return std::numeric_limits<double>::quiet_NaN();

  // Y2K: Handle 2 digit years.
  if (year >= 0 && year < 100) {
    if (year < 50)
      year += 2000;
    else
      year += 1900;
  }

  return ymdhmsToSeconds(year, month + 1, day, hour, minute, second) *
         msPerSecond;
}

double parseDateFromNullTerminatedCharacters(const char* dateString) {
  bool haveTZ;
  int offset;
  double ms = parseDateFromNullTerminatedCharacters(dateString, haveTZ, offset);
  if (std::isnan(ms))
    return std::numeric_limits<double>::quiet_NaN();

  // fall back to local timezone
  if (!haveTZ) {
    double utcOffset = calculateUTCOffset();
    double dstOffset = calculateDSTOffset(ms, utcOffset);
    offset = static_cast<int>((utcOffset + dstOffset) / msPerMinute);
  }
  return ms - (offset * msPerMinute);
}

// See http://tools.ietf.org/html/rfc2822#section-3.3 for more information.
String makeRFC2822DateString(unsigned dayOfWeek,
                             unsigned day,
                             unsigned month,
                             unsigned year,
                             unsigned hours,
                             unsigned minutes,
                             unsigned seconds,
                             int utcOffset) {
  StringBuilder stringBuilder;
  stringBuilder.append(weekdayName[dayOfWeek]);
  stringBuilder.append(", ");
  stringBuilder.appendNumber(day);
  stringBuilder.append(' ');
  stringBuilder.append(monthName[month]);
  stringBuilder.append(' ');
  stringBuilder.appendNumber(year);
  stringBuilder.append(' ');

  appendTwoDigitNumber(stringBuilder, hours);
  stringBuilder.append(':');
  appendTwoDigitNumber(stringBuilder, minutes);
  stringBuilder.append(':');
  appendTwoDigitNumber(stringBuilder, seconds);
  stringBuilder.append(' ');

  stringBuilder.append(utcOffset > 0 ? '+' : '-');
  int absoluteUTCOffset = abs(utcOffset);
  appendTwoDigitNumber(stringBuilder, absoluteUTCOffset / 60);
  appendTwoDigitNumber(stringBuilder, absoluteUTCOffset % 60);

  return stringBuilder.toString();
}

double convertToLocalTime(double ms) {
  double utcOffset = calculateUTCOffset();
  double dstOffset = calculateDSTOffset(ms, utcOffset);
  return (ms + utcOffset + dstOffset);
}

}  // namespace WTF