If you think of your timing as a number line with TDC in the center, representing zero, and 180 degrees on either side, anything to the left would be negative, or before TDC. So 'timing' is negative in this case. In newer computers, 'timing advance' is calculated and displayed as a positive number. However, it still represents the same value. I hope this description makes sense to you guys... At least it makes sense in my head...