All seems to be excellent, except handling of leap-seconds.  
That doesn't exist yet.

The current source is designed to compile flawlessly under all platforms
supported under gcc 3.*/4.* and up, and probably much 2.*.

To compile aptimelib.o, for linking with other code:
on solid POSIX,  gcc -c aptime.c -D_POSIX
on generic Unix, gcc -c aptime.c -D_unix
on Cygwin,       gcc -c aptime.c -D_WIN32
on MingW,        gcc -c aptime.c

To compile the test driver:
gcc -o aptl_test aptl_test.c aptimelib.c -D_POSIX
gcc -o aptl_test aptl_test.c aptimelib.c -D_unix
gcc -o aptl_test aptl_test.c aptimelib.c -D_WIN32
gcc -o aptl_test aptl_test.c aptimelib.c

Changes from 0.5 --> 0.6 are just code cleanup, simplifying
versioning and separating the test driver from aptimelib.c.

0.7 will add some interface functions to simplify overall usage.