Well, if you really know the latitude of the place where you, I guess you can calculate it. Since the Earth's axe leans a little bit (27°23' angle), from the Northern Emisphere you see the Sun moving trough the sky from East to West curving to South (E–>S–>W in curved motion; curve to North if you're in the Southern Emisphere), that it reaches at about mid-day. You can use the compass to find the position of the Sun and then approximatively calculate the hour (don't forget to count also about solar/legal time!); you need to know the latitude because how much the Sun curves depends on it. If you draw down the situation, you should easily get to with some simple calculations; it would me much complicated my explaining them here without any scheme. However I don't know if there are other ways, I just infered this from my astronomy&geography poor skills, so wait for other answers; by the way I can exactly use an analogical watch as a compass (if there's sun...!). Hope this helps.