title: Convert string time to UNIX timestamp
body: <p>I have a string like <code>2013-05-29T21:19:48Z</code>. I'd like to convert it to the number of seconds since 1 January 1970 (the UNIX epoch), so that I can save it using just 4 bytes (or maybe 5 bytes, to avoid the year 2038 problem). How can I do that in a portable way? (My code has to run both on Linux and Windows.)</p>
  
  <p>I can get the date parts out of the string, but I don't know how to figure out the number of seconds. I tried looking at the <a href="http://en.cppreference.com/w/cpp/chrono" rel="noreferrer">documentation of date and time utilities in C++</a>, but I didn't find anything.</p>