Time_iso8601 is a project mainly written in C++ and C, it's free.
The ISO8601 time format.
=== The ISO8601 time format for C++ Copyright 2010 Toshiyuki Terashita.
sample: // t is initialized by time(NLLL). TimeIso8601 t(NULL);
// Convert to string. ex: "2010-09-29T12:24:56+0900" // Set to the time. std::string str = t;
// Set to time of "2000-01-04T12:00:00+0900". t = "2000-01-04T12:00:00-0900"
// Convert to time_t time_t tm = t;