Add March and September as months which can have leap seconds.

Obtained from "ITU-R, "Recommendation ITU-R TF.460-6 - Standard-frequency and time-signal emissions", February 2002."
via http://tools.ietf.org/html/draft-ietf-avtcore-leap-second-05#ref-6
Sort and prune #includes
Fold two lines into one.
Indent a comment.
Replace a bunch of if/else constructs with ternary expressions.