The attached patches follow up on a suggestion by Stefan a few weeks 
ago, by adding support for a new Lisp timestamp format (TIMESTAMP . 
FREQUENCY), where TIMESTAMP is an integer that counts clock ticks and 
FREQUENCY is a positive integer that counts ticks per second. For 
brevity the documentation says (TICKS . HZ) instead of (TIMESTAMP . 
FREQUENCY).

Although current-time and similar functions continue to return the (HI 
LO US PS) format, the idea is that Emacs eventually should switch to 
(TICKS . HZ) as it is a better match for an Emacs with bignums (among 
other things, it does not lose information), and in the meantime we can 
better document that the Lisp timestamp format has changed in the past 
and can change in the future, and that user code should not depend on 
the exact timestamp format.