Meniscus clock

A double walled glass sits next to a card on the corner of your desk.
It appears to be slightly more than half full, but a glance inside shows that it is entirely empty as all of the liquid is encapsulated between the inner and outer walls.

A closer look reveals tiny inverted markings etched at even intervals up the inside which, when viewed from the outside, are magnified by the meniscus of the water ever so slowly creeping up the sides of the vessel.
These markings are hour and minute increments, increasing in number until they reach 12:00, the time the card says is the lunch reservation your boss made for your birthday.

awwwwwwwwww.