The 645 clock was a huge box, 8 foot refrigerator size, containing a clock accurate to a microsecond. It hooked into the system as a "passive device," meaning that it looked like a bank of memory. Memory reads from a port with a clock on it returned the time in microseconds since 0000 GMT Jan 1, 1901. (52-bit register) The clock guaranteed that no two readings were the same. It had a real-time alarm register also. Inside there was a crystal in an oven, all kinds of ancient electronics.
-- from a description of the Multics implementation on the the GE-645
That's funny. It seems like serious overkill just to make unique timestamps, even for Multics. :)