So I want to creata a time stamp (as a string) with the format HH:MM:SS in C++. I use std::chrono to get a unix time stamp and then calculate the hours, minutes and seconds.
// Get unix time stamp in seconds.
const auto unix_time_stamp = std::chrono::system_clock::now();
long long seconds_since_epoch = std::chrono::duration_cast<std::chrono::seconds>(unix_time_stamp.time_since_epoch()).count();
// Calculate current time (hours, minutes, seconds).
uint8_t hours = (seconds_since_epoch % 86400) / 3600;
uint8_t minutes = (seconds_since_epoch % 3600) / 60;
uint8_t seconds = (seconds_since_epoch % 60);
// Create strings for hours, minutes, seconds.
std::string hours_string = std::to_string(hours);
std::string minutes_string = std::to_string(minutes);
std::string seconds_string = std::to_string(seconds);
// Check if the number is only one digit. If it is, add a 0 in the beginning (5:3:9 --> 05:03:09).
if(hours_string.size() == 1)
{
hours_string = "0" + hours_string;
}
if(minutes_string.size() == 1)
{
minutes_string = "0" + minutes_string;
}
if(seconds_string.size() == 1)
{
seconds_string = "0" + seconds_string;
}
// Append to a final string.
std::string time_stamp = hours_string + ":" + minutes_string + ":" + seconds_string;
This is all working fine and great but there is one big problem: time zones.
With this way, I'm only calculating the time stamp for GMT. Is there any easy, fast and, most importantly, portable way to get the "offset" in seconds or minutes or hours for your system's time zone? By "portable" I mean platform-independent.
Please note: I know you can do all of this more easily with std::strftime and so on, but I really want to implement this by myself.
Some implementations of std::tm will contain a member that has the local offset as a member. ... But it isn't portable.
One trick is to take your seconds_since_epoch, and either assign it to a std::time_t, or just make its type std::time_t in the first place instead of long long.
... Oh, wait that isn't quite portable. Some platforms still use a 32 bit time_t. But assuming a 64 bit time_t ...
Then use localtime to get a std::tm:
std::tm tm = *localtime(&seconds_since_epoch);
This isn't officially portable because system_clock and time_t aren't guaranteed to have the same epoch. But in practice they do.
Now take the {year, month, day, hour, minute, second} fields out of the tm and compute a "local epoch". The hard part of this computation is converting the {year, month, day} part into a count of days. You can use days_from_civil from here to do that computation efficiently. Be sure to take the weird offsets into account for tm_year and tm_mon when doing this.
After you get this then subtract seconds_since_epoch from it:
auto offset = local_epoch - seconds_since_epoch;
This is your signed UTC offset in seconds. Positive is east of the prime meridian.
In C++20 this simplifies down to:
auto offset = std::chrono::current_zone()->get_info(system_clock::now()).offset;
and offset will have type std::chrono::seconds.
You can get a free, open-source preview of this here. It does require some installation.
In using the tm structure in c++, i get values of tm_wday that are large integers (e.g. 4199040, much larger than the expected 0-6 range: ). Why would this happen? All the other values such as the year, month etc. are correct.
I have seen previous questions where the weekday seems to be calculated wrongly,i.e. is a different value within the 0-6 range than expected due to time zone differences etc. but I am baffled as to why i would get such a big number instead? It doesn't seem to be a memory location either (not a hex format number).
#include <stdio.h>
#include <iostream>
#include <time.h>
struct tm get_time(std::string timestamp_string = "2019.08.16D11:00:00"){
struct tm tm;
int hh, mm;
int MM, DD, YY;
float ss;
const char * timestamp = timestamp_string.c_str();
if (sscanf(timestamp,"%d.%d.%dD%d:%d:%f", &YY,&MM, &DD,&hh, &mm,&ss) != 6) std::cout<<"oops";
tm.tm_year = YY - 1900; // Years from 1900
tm.tm_mon = MM - 1; // Months form January
tm.tm_mday = DD;
tm.tm_hour = hh;
tm.tm_min = mm;
tm.tm_sec = ss;
tm.tm_isdst = 0;
return tm;
}
int main(){
struct tm tm = get_time("2019.08.16D11:00:00");
std::cout<<"Year is: "<<tm.tm_year<<std::endl; //119 - is correct
std::cout<<"Month since Jan is: "<<tm.tm_mon<<std::endl; //7 - is correct
std::cout<<"Weekday is: "<<tm.tm_wday<<std::endl;//4199040- why is this so large?
return 0;
}
Looking around some more, it is expected that after a tm structure is defined, you run the function mktime() with a reference to the instance in order to update derived values such as tm_wday. So the fixed main() function should be:
int main(){
struct tm tm = get_time("2019.08.16D11:00:00");
mktime(&tm); //needs to be called to update derived values such as tm_wday
std::cout<<"Year is: "<<tm.tm_year<<std::endl; //119 - is correct
std::cout<<"Month since Jan is: "<<tm.tm_mon<<std::endl; //7 - is correct
std::cout<<"Weekday is: "<<tm.tm_wday<<std::endl;//shows 5 now - is correct
return 0;
}
Why should it be any other value?
You never set it to anything.
So, it retains its initial value, which is unspecified since you never initialised the tm.
Therefore, it's not just "a large integer": your whole program has undefined behaviour as a result of attempting to read this unspecified value.
If you expected it to be automatically set to the appropriate weekday index for the values you entered, that's not how it works. The tm is just a collection of values, not a function.
However, you can call mktime to do what you need:
The mktime() function modifies the fields of the tm structure as follows: tm_wday and tm_yday are set to values determined from the contents of the other fields; if structure members are outside their valid interval, they will be normalized (so that, for example, 40 October is changed into 9 November); tm_isdst is set (regardless of its initial value) to a positive value or to 0, respectively, to indicate whether DST is or is not in effect at the specified time. Calling mktime() also sets the external variable tzname with information about the current timezone.
(ref)
I have a program that was originally written for Linux, but I now have a requirement to get it running on Solaris 10.
Part of this program uses the timegm function to convert a struct tm into a time_t epoch seconds value. The input time is referenced to UTC.
Trying to compile this program on Solaris, it fails because timegm cannot be found. After some googling I realized that this function has been removed from Solaris a long time ago (and even the Linux manpage recommends against using it, because it isn't standardized).
However I have so far not been able to find an alternative function, that takes a struct tm referenced to UTC and converts to epoch time. Most references I found on the net recommend using mktime, however that function interprets the inputs with reference to the system local time zone.
Note that I do not wish to use tzset to force the timezone to UTC, as that would have other side effects on the program.
So my question is: how can I convert a struct tm broken down time value, expressed with respect to UTC, into an epoch time, in the absence of timegm?
The program is written in C++ so I'm not limited to C solutions, although I would prefer not to embark on a wholesale rewrite to use some additional time library.
You could use days_from_civil which is described here in detail
// Returns number of days since civil 1970-01-01. Negative values indicate
// days prior to 1970-01-01.
// Preconditions: y-m-d represents a date in the civil (Gregorian) calendar
// m is in [1, 12]
// d is in [1, last_day_of_month(y, m)]
// y is "approximately" in
// [numeric_limits<Int>::min()/366, numeric_limits<Int>::max()/366]
// Exact range of validity is:
// [civil_from_days(numeric_limits<Int>::min()),
// civil_from_days(numeric_limits<Int>::max()-719468)]
template <class Int>
constexpr
Int
days_from_civil(Int y, unsigned m, unsigned d) noexcept
{
static_assert(std::numeric_limits<unsigned>::digits >= 18,
"This algorithm has not been ported to a 16 bit unsigned integer");
static_assert(std::numeric_limits<Int>::digits >= 20,
"This algorithm has not been ported to a 16 bit signed integer");
y -= m <= 2;
const Int era = (y >= 0 ? y : y-399) / 400;
const unsigned yoe = static_cast<unsigned>(y - era * 400); // [0, 399]
const unsigned doy = (153*(m + (m > 2 ? -3 : 9)) + 2)/5 + d-1; // [0, 365]
const unsigned doe = yoe * 365 + yoe/4 - yoe/100 + doy; // [0, 146096]
return era * 146097 + static_cast<Int>(doe) - 719468;
}
to convert the {year, month, day} triple in the tm to a count of days since the epoch (1970-01-01). Be careful when converting these fields from tm for their eccentricities (e.g. tm_year + 1900).
Multiply this count of days by 86400 and add to that the {hours, minutes, seconds} data from the tm (each converted to seconds).
And you're done. Don't worry about leap seconds, timegm didn't worry about them either. If you're really concerned about leap seconds I have a C++11/14 solution available to deal with that, but I'm guessing that is more than you want to get into.
Don't be put off by the C++14 syntax shown above. It is trivial to convert this algorithm to C (or any other language for that matter).
Per the POSIX standard for tzset():
SYNOPSIS
#include <time.h>
extern int daylight;
extern long timezone;
extern char *tzname[2];
void tzset(void);
...
The tzset() function also shall set the external variable daylight
to 0 if Daylight Savings Time conversions should never be applied for
the timezone in use; otherwise, non-zero. The external variable
timezone shall be set to the difference, in seconds, between
Coordinated Universal Time (UTC) and local standard time.
You should be able to call tzset() to set the value in timezone, then use mktime() to get the time in the current timezone, then apply the difference in the timezone variable to to the result from mktime() to convert that result to UTC.
I don't have access to Solaris right now to test that.
I am trying to take the difference of two dates by first reading the local time saving the tm structure and going to sleep for 5 seconds and read another local time and saving to another tm structure. I was hoping once I take the differences of the two dates to get a value 5 or greater. However, I am getting 0.
I get the correct result if comment out the following lines:
oldyear.tm_year = oldyear.tm_year + 1900;
oldyear.tm_mon = oldyear.tm_mon + 1;
newyear.tm_year = newyear.tm_year + 1900;
newyear.tm_mon = newyear.tm_mon + 1;
My code:
void timeTest()
{
time_t now;
struct tm newyear, oldyear;
double seconds;
time(&now); /* get current time; same as: now = time(NULL) */
oldyear = *localtime(&now);
oldyear.tm_year = oldyear.tm_year + 1900;
oldyear.tm_mon = oldyear.tm_mon + 1;
int epoch1 = mktime(&oldyear);
sleep(5);
time(&now); /* get current time; same as: now = time(NULL) */
newyear = *localtime(&now);
newyear.tm_year = newyear.tm_year + 1900;
newyear.tm_mon = newyear.tm_mon + 1;
int epoch2 = mktime(&newyear);
seconds = difftime(mktime(&newyear),mktime(&oldyear));
printf ("%.f seconds since new year in the current timezone.\n", seconds);
}
If I compile this on Linux, a 64-bit system, I get the output
5 seconds since new year in the current timezone.
However, if I compile for 32-bit system,
% gcc -m32 test2.c
% ./a.out
0 seconds since new year in the current timezone.
Note that mktime expects that the year is 1900-based and month 0-based, so the adjustment you do is incorrect and might cause overflow on 32-bit computers. What your code does is calculate the difference of 2 points of time on date 3914-08-28 - on 32-bit systems the time_t usually is 32 bits, and the largest date representable is 03:14:07 UTC on Tuesday, 19 January 2038 aka Y2K38 jf signed time_t is used.
On errors -1 is returned:
If the specified broken-down time cannot be represented as calendar
time (seconds since the Epoch), mktime() returns (time_t) -1 and does
not alter the members of the broken-down time structure.
Thus if you print out epoch and epoch2 I could bet you get -1 for both these timestamps.
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?
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;
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);
}
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().
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.
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;
}
Use _mkgmtime, it takes care of everything.
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;
}
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
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
;
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
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.
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.