In ZODB 4, the cache eviction may be done quite differently. The version number may be a simple int. The last mod time will not be provided for each object; instead, users will need to define this themselves if they care about it. If they define it themselves, they'd probably use a DateTime object, but we'd care much less about how small it is.
In that case, I take back everything I've said about Jim Fulton's requirements. I'm quite sure that in the past he said he needed a very lightweight date/time object, but from what you say it appears this need has disappeared.
--Guido van Rossum (home page: http://www.python.org/~guido/)