Usually a tiny 0.5mm difference makes no difference, but not in this case. The CR1220 battery has the right width, but the thickness was off, making the watch act up intermittently. Unknown to me was the cause, so I wedged the battery for years. Why did not Cardinal write the battery type on the inside of the watch case? The CR1225 is just a smidge thicker, 0.5mm, which makes all the difference.
Further, the CR1225 has a 25% higher mAh rating, so should last longer.