Ex post facto doesn't necessarily come into play with statute of limitations though. Ex post facto prevents a once-legal situation from becoming illegal, so as (extreme hyperbole here) to prevent the government from outlawing reading books tomorrow, and punishing everybody who read a book today. There are additional sideways extensions to ex post facto that prevent milder penalties from being increased, and that sort of thing (e.g., so that if they'd arrested Capone on Jaywalking, they can't increase the penalties to be life-long prison sentences or what have you), but there aren't any provisions in the actual Constitution that prohibit adjustments of statute.

That said Stogner v. California is the relevant case here, which ruled that express act as unconstitutional, despite its omission from the Constitution proper, but even in that case, it was overturned, then upheld, then overturned again, so its provisions aren't so obvious that some clever wording couldn't complicate the issue further enough into more vagueness. Lest we forget that the government can, and very often does pass laws that seem "clearly" unconstitutional. Even further, sometimes those laws are upheld.