At Jane Street we have recently spend a lot of time trying to get a fast gettimeofday. I saw lots of references in various postgres hacker threads related to a lack of such a facility so ....
The culmination of those efforts can be read here:
it's all OCaml but the code is mostly imperative and very well documented. In particular we made an effort to document our assumption. There are a few which are ocaml specific. But a lot of the lessons we have learned here should be applicable to postgres.
Hope this will be useful,
Cheers,
Bene
PS: We are releasing our code under the Apache license so you should feel free to reuse the ideas.