The Imperium ignores individual planetary time, clocking everything on Sylean (e.g. Imperial) time. This drives the locals crazy, of course. Particularly because it's merely a legal fiction (since there is something in quantum mechanics - ask somebody with the math exactly what - which makes keeping accurate time on an interstellar scale impossible).

But the Imperium doesn't care about that. In fact, there is a bit of canon which makes local compliance with the Imperial calendar for all official uses connected with the Empire one of the signs of whether or not a particular world is successfully integrating into the Imperium. There is also a tradition among many grognards (I don't *think* it's actually canon) that given with it's authority to investigate all official interactions between Imperial and local bodies, the Imperial Office of Calendar Compliance is one of the more effective of the various clandestine internal Imperial spy agencies.