You need to be very careful with stepper motor ratings. 12V is the maximum peak voltage you may apply through a chopping-type driver (not the sort of chip you are using). The datasheet says the rated current for that stepper is 400mA and the resistance is 4 ohms per phase. So the maximum continuous voltage you may apply is 0.4 * 4 = 1.6V per phase. The SN754410 has a typical voltage drop of 2.1V @ 0.5A, so the maximum motor voltage you should feed to it is about 1.6 + 2.1 = 3.7V. Any more and you will overheat the stepper.
I suggest you use a 5V regulated wall wart (e.g. USB power adapter) rated at at least 1A, and put a 3.3 ohm or 4.7 ohm 1W resistor in series with each motor winding. In fact, for a clock you need very little torque or speed, so you can probably get away with 12 ohm resistors (which will reduce the current per phase to about 200mA), then you can use 0.5W resistors and a 0.5A power supply.