C++ Converting a time string to seconds from the e

2019-01-14 13:27发布

I have a string with the following format:

2010-11-04T23:23:01Z

The Z indicates that the time is UTC.
I would rather store this as a epoch time to make comparison easy.

What is the recomended method for doing this?

Currently (after a quck search) the simplist algorithm is:

1: <Convert string to struct_tm: by manually parsing string>
2: Use mktime() to convert struct_tm to epoch time.

// Problem here is that mktime uses local time not UTC time.

10条回答
在下西门庆
2楼-- · 2019-01-14 13:41

You could utilize the boost::date_time and write a small manual parser (probably regexp-based) for your strings.

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贼婆χ
3楼-- · 2019-01-14 13:43

X/Open provides a global timezone variable which indicates the number of seconds that local time is behind UTC. You can use this to adjust the output of mktime():

#define _XOPEN_SOURCE
#include <stdio.h>
#include <time.h>

/* 2010-11-04T23:23:01Z */
time_t zulu_time(const char *time_str)
{
    struct tm tm = { 0 };

    if (!strptime(time_str, "%Y-%m-%dT%H:%M:%SZ", &tm))
        return (time_t)-1;

    return mktime(&tm) - timezone;
}
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聊天终结者
4楼-- · 2019-01-14 13:50

Problem here is that mktime uses local time not UTC time.

Linux provides timegm which is what you want (i.e. mktime for UTC time).

Here is my solution, which I forced to only accept "Zulu" (Z timezone). Note that strptime doesn't actually seem to parse the time zone correctly, even though glib seems to have some support for that. That is why I just throw an exception if the string doesn't end in 'Z'.

static double EpochTime(const std::string& iso8601Time)
{
    struct tm t;
    if (iso8601Time.back() != 'Z') throw PBException("Non Zulu 8601 timezone not supported");
    char* ptr = strptime(iso8601Time.c_str(), "%FT%T", &t);
    if( ptr == nullptr)
    {
        throw PBException("strptime failed, can't parse " + iso8601Time);
    }
    double t2 = timegm(&t); // UTC
    if (*ptr)
    {
        double fraction = atof(ptr);
        t2 += fraction;
    }
    return t2;
}
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冷血范
5楼-- · 2019-01-14 13:51

Problem here is that mktime uses local time not UTC time.

How about just computing the time difference between UTC and local time, then adding it to the value returned by mktime?

time_t local = time(NULL),
       utc   = mktime(gmtime(&local));
int    diff  = utc - local;
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霸刀☆藐视天下
6楼-- · 2019-01-14 13:54

New answer to an old question. Rationale for new answer: In case you want to use <chrono> types to solve a problem like this.

In addition to C++11/C++14, you'll need this free, open source date/time library:

#include "tz.h"
#include <iostream>
#include <sstream>

int
main()
{
    std::istringstream is("2010-11-04T23:23:01Z");
    is.exceptions(std::ios::failbit);
    date::sys_seconds tp;
    date::parse(is, "%FT%TZ", tp);
    std::cout << "seconds from epoch is " << tp.time_since_epoch().count() << "s\n";
}

This program outputs:

seconds from epoch is 1288912981s

If the parse fails in any way, an exception will be thrown. If you would rather not throw exceptions, don't is.exceptions(std::ios::failbit);, but instead check for is.fail().

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在下西门庆
7楼-- · 2019-01-14 13:55

It's not an exact dup but you will find @Cubbi's answer from here useful, I wager. This specifically assumes UTC input.

Boost also support direct conversion from ISO 8601 via boost::posix_time::from_iso_string which calls boost::date_time::parse_iso_time, here again you would just strip the trailing 'Z' and treat the TZ as implicit UTC.

#include <iostream>
#include <boost/date_time.hpp>

namespace bt = boost::posix_time;

const std::locale formats[] = {
std::locale(std::locale::classic(),new bt::time_input_facet("%Y-%m-%d %H:%M:%S")),
std::locale(std::locale::classic(),new bt::time_input_facet("%Y/%m/%d %H:%M:%S")),
std::locale(std::locale::classic(),new bt::time_input_facet("%d.%m.%Y %H:%M:%S")),
std::locale(std::locale::classic(),new bt::time_input_facet("%Y-%m-%d"))};
const size_t formats_n = sizeof(formats)/sizeof(formats[0]);

std::time_t pt_to_time_t(const bt::ptime& pt)
{
    bt::ptime timet_start(boost::gregorian::date(1970,1,1));
    bt::time_duration diff = pt - timet_start;
    return diff.ticks()/bt::time_duration::rep_type::ticks_per_second;

}
void seconds_from_epoch(const std::string& s)
{
    bt::ptime pt;
    for(size_t i=0; i<formats_n; ++i)
    {
        std::istringstream is(s);
        is.imbue(formats[i]);
        is >> pt;
        if(pt != bt::ptime()) break;
    }
    std::cout << " ptime is " << pt << '\n';
    std::cout << " seconds from epoch are " << pt_to_time_t(pt) << '\n';
}
int main()
{
    seconds_from_epoch("2004-03-21 12:45:33");
    seconds_from_epoch("2004/03/21 12:45:33");
    seconds_from_epoch("23.09.2004 04:12:21");
    seconds_from_epoch("2003-02-11");
}
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