title: Converting time string into epoch time on windows
body: <p>I have an interface where I'm getting a file and the file name is the time stamp when the internal data was valid.   I'm processing the files in order and providing them to another API which takes the time as milliseconds since Jan 1st, 1970.   </p>
  
  <p>So, I get the file name in as a string in the format of YYYYMMDD_HHmmSS.sss and have to turn this into time.   I assume the processing and collection occurred in the same timezone and such and simply have to convert the characters to digits.</p>
  
  <pre><code>uint64_t foo::convertFileNameToTimestamp(const std::string&amp; filename) const
  {
      const uint64_t yyyy = atoi(filename.substr(0,4).c_str());
      const uint64_t MM = atoi(filename.substr(4,2).c_str());
      const uint64_t DD = atoi(filename.substr(6,2).c_str());
      const uint64_t HH = atoi(filename.substr(9,2).c_str());
      const uint64_t MI = atoi(filename.substr(11,2).c_str());
      const uint64_t SS = atoi(filename.substr(13,2).c_str());
      const uint64_t sss = atoi(filename.substr(16,3).c_str());
  
      // number of milliseconds in a day
      const uint64_t DAY = 24 * 60 * 60 * 1000;
      int MD = 0;
      if (MM == 2) MD = 31; // currently feb, so add all of january's days
      else if (MM == 3) MD = 31+28; // ...
      else if (MM == 4) MD = 31+28+31;
      else if (MM == 5) MD = 31+28+31+30;
      else if (MM == 6) MD = 31+28+31+30+31;
      else if (MM == 7) MD = 31+28+31+30+31+30;
      else if (MM == 8) MD = 31+28+31+30+31+30+31;
      else if (MM == 9) MD = 31+28+31+30+31+30+31+31;
      else if (MM == 10) MD = 31+28+31+30+31+30+31+31+30;
      else if (MM == 11) MD = 31+28+31+30+31+30+31+31+30+31;
      else if (MM == 12) MD = 31+28+31+30+31+30+31+31+30+31+30;
  
      // year 2000 wasn't a leap year
      uint64_t YLD = ((yyyy-1970) / 4);
      if (yyyy &gt; 2000) --YLD;
  
      uint64_t temp = sss;
      temp += SS * 1000;
      temp += MI * 60 * 1000;
      temp += HH * 60 * 60 * 1000;
      temp += (DD-1) * DAY;
      temp += (MD) * DAY;
      temp += (yyyy-1970) * 365 * DAY + YLD*DAY;
  
      return temp;
  }
  </code></pre>
  
  <p>Obviously, reinventing the wheel here.   Seems like there should be some sort of function for this.   Also.. how do I account for leap seconds?  It was annoying enough to deal with leap days.   The time stamps are all from 2015 and beyond and always will be, but I don't think I can just blindly add 26 seconds.  Eventually we will have 27 or back up to 25.  In previous functions I have validated the string to be the right format and the file to exist and all that jazz.   I'm running on windows 8.1 compilation for 64 bit using VS 2010.   </p>
  
  <p>I've looked at <a href="https://stackoverflow.com/questions/14792658/convert-epoch-time-string-to-time">Convert Epoch Time string to Time</a> which suggested <em>ctime()</em>, but it doesn't seem to deal with milliseconds in the constructor or even any of the get methods and it doesn't accept generically formatted string inputs.   I'm assuming I've got to call some time classes CTOR which will accept the file name string and then call some accessor on it to get the millisecond time since 1970 including leap seconds and such.</p>
  
  <p>I am not using boost and don't have access/permission to use it.</p>