std::mktime and timezone info

Question

I'm trying to convert a time info I reveive as a UTC string to a timestamp using std::mktime in C++. My problem is that in <ctime> / <time.h> there is no function to convert to UTC; mktime will only return the timestamp as local time.

So I need to figure out the timezone offset and take it into account, but I can't find a platform-independent way that doesn't involve porting the whole code to boost::date_time. Is there some easy solution which I have overlooked?

Answer 1

timestamp = mktime(&tm) - _timezone;

or platform independent way:

 timestamp = mktime(&tm) - timezone;

If you look in the source of mktime() on line 00117, the time is converted to local time:

seconds += _timezone;

Answer 2

I have this same problem yesterday and searching man mktime:

The functions mktime() and timegm() convert the broken-out time (in the structure pointed to by *timeptr) into a time value with the same encoding as that of the values returned by the time(3) function (that is, seconds from the Epoch, UTC). The mktime() function interprets the input structure according to the current timezone setting (see tzset(3)). The timegm() function interprets the input structure as representing Universal Coordinated Time (UTC).

In short:

You should use timegm(), instead of using mktime().

Answer 3

mktime() uses tzname for detecting timezone. tzset() initializes the tzname variable from the TZ enviroment variable. If the TZ variable appears in the enviroment but its value is empty or its value cannot be correctly interpreted, UTC is used.

A portable (not threadsafe) version according to the timegm manpage

   #include <time.h>
   #include <stdlib.h>

   time_t
   my_timegm(struct tm *tm)
   {
       time_t ret;
       char *tz;

       tz = getenv("TZ");
       setenv("TZ", "", 1);
       tzset();
       ret = mktime(tm);
       if (tz)
           setenv("TZ", tz, 1);
       else
           unsetenv("TZ");
       tzset();
       return ret;
   }

Eric S Raymond has a threadsafe version published in his article Time, Clock, and Calendar Programming In C

time_t my_timegm(register struct tm * t)
/* struct tm to seconds since Unix epoch */
{
    register long year;
    register time_t result;
#define MONTHSPERYEAR   12      /* months per calendar year */
    static const int cumdays[MONTHSPERYEAR] =
        { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };

    /*@ +matchanyintegral @*/
    year = 1900 + t->tm_year + t->tm_mon / MONTHSPERYEAR;
    result = (year - 1970) * 365 + cumdays[t->tm_mon % MONTHSPERYEAR];
    result += (year - 1968) / 4;
    result -= (year - 1900) / 100;
    result += (year - 1600) / 400;
    if ((year % 4) == 0 && ((year % 100) != 0 || (year % 400) == 0) &&
        (t->tm_mon % MONTHSPERYEAR) < 2)
        result--;
    result += t->tm_mday - 1;
    result *= 24;
    result += t->tm_hour;
    result *= 60;
    result += t->tm_min;
    result *= 60;
    result += t->tm_sec;
    if (t->tm_isdst == 1)
        result -= 3600;
    /*@ -matchanyintegral @*/
    return (result);
}

Answer 4

If you are trying to do this in a multithreaded program and don't want to deal with locking and unlocking mutexes (if you use the environment variable method you'd have to), there is a function called timegm that does this. It isn't portable, so here is the source: http://trac.rtmpd.com/browser/trunk/sources/common/src/platform/windows/timegm.cpp

int is_leap(unsigned y) {
    y += 1900;
    return (y % 4) == 0 && ((y % 100) != 0 || (y % 400) == 0);
}

time_t timegm (struct tm *tm)
{
    static const unsigned ndays[2][12] = {
        {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
        {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
    };
    time_t res = 0;
    int i;

    for (i = 70; i < tm->tm_year; ++i)
        res += is_leap(i) ? 366 : 365;

    for (i = 0; i < tm->tm_mon; ++i)
        res += ndays[is_leap(tm->tm_year)][i];
    res += tm->tm_mday - 1;
    res *= 24;
    res += tm->tm_hour;
    res *= 60;
    res += tm->tm_min;
    res *= 60;
    res += tm->tm_sec;
    return res;
}

Answer 5

Use _mkgmtime, it takes care of everything.

Answer 6

Here is a simple, tested, hopefully portable piece of code converting from struct tm to seconds since the beginning of an adjustable UTC year, without temporary change of time zone.

// Conversion from UTC date to second, signed 64-bit adjustable epoch version.
// Written by François Grieu, 2015-07-21; public domain.

#include <time.h>                   // needed for struct tm
#include <stdint.h>                 // needed for int_least64_t
#define MY_EPOCH    1970            // epoch year, changeable
typedef int_least64_t my_time_t;    // type for seconds since MY_EPOCH

// my_mktime  converts from  struct tm  UTC to non-leap seconds since 
// 00:00:00 on the first UTC day of year MY_EPOCH (adjustable).
// It works since 1582 (start of Gregorian calendar), assuming an
// apocryphal extension of Coordinated Universal Time, until some
// event (like celestial impact) deeply messes with Earth.
// It strive to be strictly C99-conformant.
//
// input:   Pointer to a  struct tm  with field tm_year, tm_mon, tm_mday,
//          tm_hour, tm_min, tm_sec set per  mktime  convention; thus
//          - tm_year is year minus 1900;
//          - tm_mon is [0..11] for January to December, but [-2..14] 
//            works for November of previous year to February of next year;
//          - tm_mday, tm_hour, tm_min, tm_sec similarly can be offset to
//            the full range [-32767 to 32767].
// output:  Number of non-leap seconds since beginning of the first UTC
//          day of year MY_EPOCH, as a signed at-least-64-bit integer.
//          The input is not changed (in particular, fields tm_wday,
//          tm_yday, and tm_isdst are unchanged and ignored).
my_time_t my_mktime(const struct tm * ptm) {
    int m, y = ptm->tm_year+2000;
    if ((m = ptm->tm_mon)<2) { m += 12; --y; }
// compute number of days within constant, assuming appropriate origin
#define MY_MKTIME(Y,M,D) ((my_time_t)Y*365+Y/4-Y/100*3/4+(M+2)*153/5+D)
    return ((( MY_MKTIME( y           ,  m, ptm->tm_mday)
              -MY_MKTIME((MY_EPOCH+99), 12, 1           )
             )*24+ptm->tm_hour)*60+ptm->tm_min)*60+ptm->tm_sec;
#undef MY_MKTIME // this macro is private
    }

Key observations allowing great simplification compared to the code in this and that answers:

• numbering months from March, all months except the one before that origin repeat with a cycle of 5 months totaling 153 days alternating 31 and 30 days, so that, for any month, and without consideration for leap years, the number of days since the previous February can be computed (within a constant) using addition of an appropriate constant, multiplication by 153 and integer division by 5;
• the correction in days accounting for the rule for leap year on years multiple-of-100 (which by exception to the multiple-of-4 rules are non-leap except if multiple of 400) can be computed (within a constant) by addition of an appropriate constant, integer division by 100, multiplication by 3, and integer division by 4;
• we can compute correction for any epoch using the same formula we use in the main computation, and can do this with a macro so that this correction is computed at compilation time.

---

Here is another version not requiring 64-bit support, locked to 1970 origin.

// Conversion from UTC date to second, unsigned 32-bit Unix epoch version.
// Written by François Grieu, 2015-07-21; public domain.

#include <time.h>                   // needed for struct tm
#include <limits.h>                 // needed for UINT_MAX
#if UINT_MAX>=0xFFFFFFFF            // unsigned is at least 32-bit
typedef unsigned      my_time_t;    // type for seconds since 1970
#else
typedef unsigned long my_time_t;    // type for seconds since 1970
#endif

// my_mktime  converts from  struct tm  UTC to non-leap seconds since 
// 00:00:00 on the first UTC day of year 1970 (fixed).
// It works from 1970 to 2105 inclusive. It strives to be compatible
// with C compilers supporting // comments and claiming C89 conformance.
//
// input:   Pointer to a  struct tm  with field tm_year, tm_mon, tm_mday,
//          tm_hour, tm_min, tm_sec set per  mktime  convention; thus
//          - tm_year is year minus 1900
//          - tm_mon is [0..11] for January to December, but [-2..14] 
//            works for November of previous year to February of next year
//          - tm_mday, tm_hour, tm_min, tm_sec similarly can be offset to
//            the full range [-32767 to 32768], as long as the combination
//            with tm_year gives a result within years [1970..2105], and
//            tm_year>0.
// output:  Number of non-leap seconds since beginning of the first UTC
//          day of year 1970, as an unsigned at-least-32-bit integer.
//          The input is not changed (in particular, fields tm_wday,
//          tm_yday, and tm_isdst are unchanged and ignored).
my_time_t my_mktime(const struct tm * ptm) {
    int m, y = ptm->tm_year;
    if ((m = ptm->tm_mon)<2) { m += 12; --y; }
    return ((( (my_time_t)(y-69)*365u+y/4-y/100*3/4+(m+2)*153/5-446+
        ptm->tm_mday)*24u+ptm->tm_hour)*60u+ptm->tm_min)*60u+ptm->tm_sec;
    }

Answer 7

mktime assumes that the date value is in the local time zone. Thus you can change the timezone environment variable beforehand (setenv) and get the UTC timezone.

Windows tzset

Can also try looking at various home-made utc-mktimes, mktime-utcs, etc.

Answer 8

A solution with little coding and portable, as it only uses mktime:

The parsed time has to be in struct tm tm. if you use c++11, you might want to use std::get_time for parsing. It parses most time strings!

Before calling mktime() be sure tm.tm_isdst is set to zero, then mktime does not adjust for daylight savings,

// find the time_t of epoch, it is 0 on UTC, but timezone elsewhere
// If you newer change timezone while program is running, you only need to do this once
// if your compiler(VS2013) rejects line below, zero out tm yourself (use memset or "=0" on all members)
struct std::tm epoch = {};
epoch.tm_mday = 2; // to workaround new handling in VC, add a day
epoch.tm_year = 70;
time_t offset = mktime(&epoch) - 60*60*24; // and subtract it again

// Now we are ready to convert tm to time_t in UTC.
// as mktime adds timezone, subtracting offset(=timezone) gives us the right result
result = mktime(&tm)-offset

Edit based on comment from @Tom

Answer 9

As other answers note, mktime() (infuriatingly) assumes the tm struct is in the local timezone (even on platforms where tm has a tm_gmtoff field), and there is no standard, cross platform way to interpret your tm as GMT.

The following, though, is reasonably cross platform—it works on macOS, Windows (at least under MSVC), Linux, iOS, and Android.

tm some_time{};
... // Fill my_time

const time_t utc_timestamp =
    #if defined(_WIN32)
        _mkgmtime(&some_time)
    #else // Assume POSIX
        timegm(&some_time)
    #endif
;

Answer 10

The tm structure used by mktime has a timezone field.
What happens if you put 'UTC' into the timzone field?

http://www.delorie.com/gnu/docs/glibc/libc_435.html

Answer 11

I've just been trying to figure out how to do this. I'm not convinced this solution is perfect (it depends on how accurately the runtime library calculates Daylight Savings), but it's working pretty well for my problem.

Initially I thought I could just calculate the difference between gmtime and localtime, and add that on to my converted timestamp, but that doesn't work because the difference will change according to the time of year that the code is run, and if your source time is in the other half of the year you'll be out by an hour.

So, the trick is to get the runtime library to calculate the difference between UTC and local time for the time you're trying to convert.

So what I'm doing is calculating my input time and then modifying that calculated time by plugging it back into localtime and gmtime and adding the difference of those two functions:

std::tm     tm;

// Fill out tm with your input time.

std::time_t basetime = std::mktime( &tm );
std::time_t diff;

tm = *std::localtime( &basetime );
tm.tm_isdst = -1;
diff = std::mktime( &tm );

tm = *std::gmtime( &basetime );
tm.tm_isdst = -1;
diff -= std::mktime( &tm );

std::time_t finaltime = basetime + diff;

It's a bit of a roundabout way to calculate this, but I couldn't find any other way without resorting to helper libraries or writing my own conversion function.

Answer 12

The easy platform-independent way to convert UTC time from string to a timestamp is to use your own timegm.

Using mktime and manipulating timezone environment variables depends on correctly installed and configured TZ database. In one case some timezone links were incorrectly configured (likely side effect of trying different time server packages) which caused mktime-based algorithm to fail on that machine depending on the selected timezone and the time.

Trying to solve this problem with mktime without changing timezone is a dead end because string time (treated as local time) cannot be correctly resolved around the time when your local clock is set back one hour to turn off DST - the same string will match two points in time.

// Algorithm: http://howardhinnant.github.io/date_algorithms.html
inline int days_from_civil(int y, int m, int d) noexcept
{
    y -= m <= 2;
    int era = y / 400;
    int yoe = y - era * 400;                                   // [0, 399]
    int doy = (153 * (m + (m > 2 ? -3 : 9)) + 2) / 5 + d - 1;  // [0, 365]
    int doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;           // [0, 146096]

    return era * 146097 + doe - 719468;
}

// Converts a broken-down time structure with UTC time to a simple time representation.
// It does not modify broken-down time structure as BSD timegm() does.
time_t timegm_const(std::tm const* t)
{
    int year = t->tm_year + 1900;
    int month = t->tm_mon;          // 0-11
    if (month > 11)
    {
        year += month / 12;
        month %= 12;
    }
    else if (month < 0)
    {
        int years_diff = (11 - month) / 12;
        year -= years_diff;
        month += 12 * years_diff;
    }
    int days_since_epoch = days_from_civil(year, month + 1, t->tm_mday);

    return 60 * (60 * (24L * days_since_1970 + t->tm_hour) + t->tm_min) + t->tm_sec;
}

This solution is free from external dependencies, threadsafe, portable and fast. Let me know if you can find any issues with the code.