esr (of The Cathedral & the Bazaar fame) has just written a comprehensive guide on the many functions, structures and styles for dealing with time in C, mostly for UNIX, Linux and POSIX operating systems.
It’s especially direct in marking obsolete functionalities, stuff that shouldn’t be used, pitfalls and recommendations. I’m sure he struggled with these APIs for much longer than most developers, so his words come from experience. Moreover, his claims contain relevant references to the documentation of the standards and releases. I’m confident that if I follow this document I will start from a safe place.
The guide: Time, Clock, and Calendar Programming In C.