From: Though someone should tell them that their images don't work. Mirror Clocks
Not so common in cars was the rear vision mirror clock. They were developed in the 1920's to free up space for the growing number of instruments on the dash and to allow back seat passengers to see the time more easily. These stem wound and set mirror clocks had a few issues – the position of the mirror could move while winding, glass glare and reading the clock at night. The remedy was the pull wind cord mirror clock developed in the 1930's which used anti-glare glass, luminous hands and the option of a built-in night light.