You can do some calibration with it, but 12-15 degrees is going to be out of range or right at the edge of out of range. Will you please call our shop to discuss this, kbrindle? 315-299-3589. It does sound like you got a faulty unit, but some things we'd want to check first:
1) Is this mounted on an interior wall, not an exterior facing wall?
2) Is it mounted near a window or other place where it is in a cold draft?
While I said you may be out of range for fully calibrating with that much discrepency, you can try:
As for calibration, if you press and hold the menu button for 15 seconds or so, it will show a screen that alternates between R and A with a temperature displayed. R is the "raw" temp reading and A is the "adjusted" reading. If you press up and down buttons on this screen it will go to "TB ###" the number is a bias adjustment value. Higher values will lower the adjusted temperature, lower values will increase the adjusted temperature, roughly 7 points per degree if I remember right.
Before modifying the TB setting please write down the value it shipped with so you can revert back if needed.
If you can calibrate it to where you want it and you find the unit works otherwise (which is entirely possible, the thermistor you got may be out of normal tolerance, but the rest of the unit is perfectly fine) you can run with it that way and maybe wait until after the heating season to send it in for replacement of that one part under warranty.