if the police officer puts the wrong date of offense on the citation, is the citation valid?
Yes (but...). And when you object to it, the officer will likely simply rewrite it and issue a new one, or the solicitor will draft an accusation and try you with that. Get a lawyer to discuss if you have proper effective defenses (or strategies).
It is still valid, but if this is the kind of case you would want to try, it would be alot of fun to say nothing until the officer is being questioned on the stand. It can make them look foolish and help submarine their credibility.