You'd need one of the rare and extremely valuable sunstones! Even through cloudcover and smoke, with holding a sunstone towards the sky will tell you at what azimuth the sun has, making it possible to track both time and date! Important happenings such as solstice or mid winter "blot" can be ascertained by a knowledge and a sunstone.
The priests naturally hoard them, as it lets them keep track of the important religious dates, the mages covet them for they need precise timing for their occult rituals. Ensue chaos and war ;)