My husband installed an Intermatic T104 208-277V timer to our pool pump. However the clock on the timer is not advancing. Can anyone tell me what we might have done wrong. The manual for the timer is of no help.:)

Power (Line) should be connected to terminals 1 and 3. The load (the pool pump) should be connected to terminals 2 and 4. You probably have those reversed.

An Electrician might verify power to timer motor terminals, You will see 2 small white wire coming in from behind timer, that is from motor, Verify it is 240 volt motor, and if 240 volts is present, you should be able to see 1 of the gears spin.
You can buy just the timer motor, There is an old style and a new style at the least, bring the motor with you to electric supply house.
Line or power needs to connect to terminals 1 and 3.