CODEWITHSUNDEEP
c++datetimestlboost
The Unknown
The Unknown

Reputation: 19694

C++ Converting a Datetime String to Epoch Cleanly

Is there a C/C++/STL/Boost clean method to convert a date time string to epoch time (in seconds)?

yyyy-mm-dd hh:mm:ss

Upvotes: 11

Views: 13612

Answers (4)

Howard Hinnant
Howard Hinnant

Reputation: 218700

In C++20 (over a decade since the question was asked :-) ), this can more easily be done with std::chrono::parse.

The question can be interpreted in a couple of ways:

Is the yyyy-mm-dd hh:mm:ss string:

  • In UTC?
  • Local?
  • Some other time zone?

In C++20, whatever the timezone the string represents, it can be correctly handled.

UTC

#include <chrono>
#include <cstdint>
#include <iostream>
#include <sstream>
#include <string>

std::int64_t
convert_utc(std::string s)
{
    std::istringstream in{std::move(s)};
    in.exceptions(std::ios::failbit);
    std::chrono::sys_seconds tp;
    in >> std::chrono::parse("%F %T", tp);
    return tp.time_since_epoch().count();
}

sys_seconds is a type-alias for a seconds-precision chrono::time_point based on system_clock. This can be parsed into using parse in a wide variety of formats. Finally the underlying integral count is extracted from the time_point and returned.

Local

std::int64_t
convert_local(std::string s)
{
    std::istringstream in{std::move(s)};
    in.exceptions(std::ios::failbit);
    std::chrono::local_seconds tp;
    in >> std::chrono::parse("%F %T", tp);
    auto tz = std::chrono::current_zone();
    return tz->to_sys(tp).time_since_epoch().count();
}

The code is similar if the string represents the computer's local time zone. The main difference is to parse into a local_seconds instead of a sys_seconds, and then use the current local time zone to convert the local_time to a sys_time.

Other time zone

std::int64_t
convert_LA(std::string s)
{
    std::istringstream in{std::move(s)};
    in.exceptions(std::ios::failbit);
    std::chrono::local_seconds tp;
    in >> std::chrono::parse("%F %T", tp);
    auto tz = std::chrono::locate_zone("America/Los_Angeles");
    return tz->to_sys(tp).time_since_epoch().count();
}

Or you can choose any other IANA time zone to do the conversion from local_seconds to sys_seconds.

Demo.

Upvotes: 0

user106014
user106014

Reputation:

On Windows platform you can do something like this if don't want to use Boost:

// parsing string
SYSTEMTIME stime = { 0 };
sscanf(timeString, "%04d-%02d-%02d %02d:%02d:%02d",
       &stime.wYear, &stime.wMonth,  &stime.wDay,
       &stime.wHour, &stime.wMinute, &stime.wSecond);

// converting to utc file time
FILETIME lftime, ftime;
SystemTimeToFileTime(&stime, &lftime);
LocalFileTimeToFileTime(&lftime, &ftime);

// calculating seconds elapsed since 01/01/1601
// you can write similiar code to get time elapsed from other date
ULONGLONG elapsed = *(ULONGLONG*)&ftime / 10000000ull;

If you prefer standard library, you can use struct tm and mktime() to do the same job.

Upvotes: 3

Reunanen
Reunanen

Reputation: 8001

See: Date/time conversion: string representation to time_t

And: [Boost-users] [date_time] So how come there isn't a to_time_t helper func?

So, apparently something like this should work:

#include <boost/date_time/posix_time/posix_time.hpp>
using namespace boost::posix_time;

std::string ts("2002-01-20 23:59:59");
ptime t(time_from_string(ts));
ptime start(gregorian::date(1970,1,1)); 
time_duration dur = t - start; 
time_t epoch = dur.total_seconds();

But I don't think it's much cleaner than Rob's suggestion: use sscanf to parse the data into a struct tm and then call mktime.

Upvotes: 11

Chris Huang-Leaver
Chris Huang-Leaver

Reputation: 6089

http://www.boost.org/doc/libs/1_39_0/doc/html/date_time.html Should do the trick.

Upvotes: 1

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