This is the 17th-century equivalent of our watch. Made out of ivory, most of these so-called diptych dials were produced in the German city of Nuremberg. This one is signed by its maker, the instrument maker Hans Truschel, who made it in 1603. The top image shows the instrument closed, in the lower image it is opened up. A small pin would be placed in the horizontal half of the instrument and from it a little string would lead to the vertical half (note the tiny red string in the lower image). When placed in the correct orientation the instrument would tell time, quite accurately, just like its better known bigger brother did, the sun dial found in gardens and attached to houses. Imagine taking this gadget out of your pocket in the 17th-century street to check what time it was. Cool.
Pic: Columbia, University Library, Smith Instrument 27-225. More on this instrument here.