Urika! I just figured out what the opposite of a clock is... here's my logic...
A clock keeps track of time in the present. As in 'Now'... So the opposite of a clock is what keeps track of time in the 'past' and in the 'future'... Drumroll please..
I therefore conclude that the opposite of a clock is a 'Calendar'
I'm so good it's almost scary!
Cheers!