Reviewing the bidding:
The new receptacle has one half controlled by the timer and fed with the red wire of the 12-3. The other half is always hot and fed with the black wire of the 12-3. The neutral side tab should not be broken. If you did break it then use two white pigtails from the 12-3 white wire to connect to the two silver screws (or buy a new receptacle).
Originally Posted by teamo
... I broke off both tabs at the existing outlet so there is no pigtailed neutral.
Correct, given that half of the existing receptacle will be fed from the new receptacle location and the other half will be fed from the existing circuit. Double check to be sure that wire ends don't stick out too far and touch things they should not, notably where they are screwed onto the receptacle and the pad for the other neutral screw, the two formerly bridged (bonded) with the tab, is very close.
With the 12-3 not connected at the panel yet both of the new receptacle halves and also the half of the existing receptacle fed from the new location should all be dead.
You should get zero volts between any two of the three: hot, neutral, ground at the new
receptacle and zero volts between hot of the existing circuit and neutral of the new circuit over at the existing receptacle. Do not use the ohms or continuity function of the meter with the power on.