new C time interface

[Nathan Myers]

[apologies if this has been seen before.]

Part of the problem with struct tm/time_t is that it works ok for 
inode times but not for other equally-important uses.

The problem with the mktime model is that conversions between civil
time and timestamps can result in 2, 1, or <1 valid conversions, and
neither the user (before the conversion) nor the library is equipped
to favor one result over another, or deal properly with nonexistent
times.  mktime offers no interface for the algorithm to report what 
it finds, so it must falsify its results.  

I hope that any interface offered to improve on mktime will expose the 
information needed to write reliable code.  If it is also "nice" for 
the common ranges where there actually is a one-to-one mapping, that's 
a bonus, but correctness should come first.

Nathan Myers
ncm at cantrip.org  http://www.cantrip.org/