Converting millisecond UTC to Human Readable Date_Time

Question

I'm struggling to figure out how to preform a conversion with boost::date_time. I want to convert a millisecond value measured from the Unix epoch (00:00, Jan 1, 1970) to a human readable string - something like: 2/13/2012 15:20:11 would be ideal.

I've tried some std C++ / boost suggestions I've seen but not had any luck just yet. Here is the code I use:

    long long ticksFromEpoch = 1329117220501;

    std::time_t tt = static_cast<time_t>(ticksFromEpoch);

    boost::posix_time::ptime dt = boost::posix_time::from_time_t(tt);

    // Create a time_zone_ptr for the desired time zone and use it to create a local_date_time
    boost::local_time::time_zone_ptr zone(new boost::local_time::posix_time_zone("UTC"));
    boost::local_time::local_date_time dt_with_zone(dt, zone);

    std::stringstream strm;

    strm.imbue(std::locale(std::cout.getloc(), new boost::local_time::local_time_facet("%Y-%m-%d %H:%M:%S"))); // 15:14
    strm << dt_with_zone;

    // Print the stream's content to the console
    std::cout << strm.str() << std::endl;

The output is: 2032-07-01 20:20:37 which is clearly incorrect. I suspect that I'm not constructing the ticksFromEpoch variable correctly but I'm not sure why. Can anyone point me in the right direction? Any help is much appreciated!!

Answer 1

time_t is usually seconds since "the epoch", rather than milliseconds. If you dont care about milliseconds you should be able to do this:

std::time_t tt = static_cast<time_t>(ticksFromEpoch/1000)

If you do care about milliseconds you can either add them back in at the end (which is tricky to get right for times like 12:00:00.001 AM )

Or you'll need to go another route. You may need to use something like this: (untested)

boost::posix_time::ptime t2(
  boost::gregorian::date(1970,boost::date_time::Jan,1),  //The epoch
  boost::posix_time::seconds( ticksFromEpoch / 1000 ) + // Number of seconds
  boost::posix_time::milliseconds( ticksFromEpoch % 1000)  // And the micros too.
  );