Re: [Boost-users] convert separate year, month, day, hour, minute, secs, ms to milliseconds since epoch

Subject: Re: [Boost-users] convert separate year, month, day, hour, minute, secs, ms to milliseconds since epoch
From: Leon Mlakar (leon_at_[hidden])
Date: 2019-03-04 23:51:40

On 05.03.2019 00:40, Leon Mlakar wrote:
> On 04.03.2019 23:17, Gavin Lambert via Boost-users wrote:
>> On 4/03/2019 09:45, Victor Yankee wrote:
>>> I am using C++14 and boost 1.64.0 (could move to newest boost), and 
>>> need to convert date and time pieces to a single value for 
>>> milliseconds since the epoch. This is what I have:
>>>
>>> int64_t msSinceEpoch(int year,int month,int day,int hour,int 
>>> minute,int second,int ms)
>>> Â Â Â Â Â Â Â Â Â Â Â Â  {
>>> Â Â Â Â Â Â Â Â Â Â Â Â  struct std::tm t;
>>>              t.tm_sec    = second;
>>>              t.tm_min    = minute;
>>>              t.tm_hour   = hour;
>>>              t.tm_mday   = day;
>>>              t.tm_mon    = month-1;
>>>              t.tm_year   = year-1900;
>>>              t.tm_isdst  = 0;
>>> Â Â Â Â Â Â Â Â Â Â Â Â  return (1000* timegm(&t))+ms; // is timegm cross-platform?
>>> Â Â Â Â Â Â Â Â Â Â Â Â  }
>>
>> timegm is not cross platform.  mktime is, but uses local time instead 
>> of UTC.
>>
>>> Is there a better way? Could not figure out how to use boost::chrono 
>>> :( Something else?
>>
>> Boost.Chrono is for time intervals, not dates.
>>
>> Boost.DateTime, however, has the ptime class which will solve this 
>> for you.
>
> I think with C++11 something like:
>
> std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::system_clock::now().time_since_epoch()).count() 
>
>
> might also do the trick.
>
> Cheers,
>
> Leon

Err, then again, it might not until C++20. C++11 embarrassingly failed 
to state what the Epoch is. So for now clocks are free to use their own 
and currently the above is useless for cross-platform applications (but 
so is timegm). I wonder how this works on Windows with VS2019?

Cheers,

Leon