Gosh, this is clever.
...the device is purely passive - it operates without electricity, and has no moving parts. Instead, the sunlight is cast through two cleverly designed masks in the shape of numbers that show the current time of day. The sundial is available in two versions, for use in either hemisphere. Placed on the inside of a south-facing window (north-facing in the southern hemisphere), the sundial can be read through the horizontal mirror. The display updates every 10 minutes, and gives a remarkably accurate record of the time during the daylight hours.Link (Thanks, manx!)