title: Why there is no inverse function for gmtime in libc?
body: <p>In libc there are two functions to convert from system time to calendar time - <code>gmtime</code> and <code>localtime</code>, but only <code>localtime</code> has inverse function - <code>mktime</code>. Why there is no inverse function for <code>gmtime</code>, and if there shouldn't be any, why <code>gmtime</code> exists?</p>