The point I don't quite get is why the power draw of a GPS receiver module (which, for just getting time, shouldn't draw all that much) would be an issue for any practical purpose?
Just use a decent RTC and update time once or twice per 24h from GPS (depending on application, a bit less frequent than this is probably still fine), that'll give you ample accuracy for maybe 20-30mA drawn for a few seconds. With "normal" reception conditions, you should get time/date from GPS just within a couple seconds, way before a "fix" has been acquired. Say, 30mA for 5s per 24h, that's an average current of 1.7µA.
Just a thought.
I've used DCF77 in past projects for this (there are equivalents in various parts of the world), but even if the power consumption of a DCF77 receiver looks attractive at first sight, the full picture needs to be known. While it draws less than a GPS receiver for sure, you'll need to power it for much, much, much longer to, maybe, if weather is ok and stars are aligned, and having to wait for a full minute cycle at each attempt, finally get the time and date. Maybe. Believe me, it will definitely draw a lot more current on average.