Star Trek: 10 Things We Now Know About The Temporal Wars
4. Temporal Law & Order
The official Temporal Prime Directive is usually the law cited that Federation citizens must follow. However, there is confusion as to when the law was written. For example, Picard did not know of it during Rasmussen's trip to the Enterprise-D in A Matter Of Time, and Bashir cited a different guideline when he is thrown into Earth's 21st Century.
Janeway was aware of the rule by 2371, though she (along with a certain James T. Kirk) had so many Temporal violations to their names, that it's hard to tell if this wasn't an isolated case. The versions of Chakotay and Kim, from 2390, knew of it, as did Admiral Janeway from 2404.
It was most likely written in the 29th Century, policed by time agents like the crew of the USS Relativity. This Federation ship seemed tasked with ensuring the flow of history continued according to the books, while the Temporal Wars would surely have been in full effect. Knowing that the Relativity, Braxton, and Ducane, were all working to thwart the Wars, draws the USS Voyager into the Temporal Wars as well - even if, on-screen at least, no mention of them was made.