TimeQuest Timing Analyzer[Setup slack analysis]

Hello Chris,For better understanding I was cross checked the same design with adding clock setup uncertainty.For this exercise I given 0.2ns for uncertainty. Here recalling the equation for setup slack analysis [ setup slack = clock arrival time -Tsu - Setup Uncertainty] Clock arrival time=22.982 ns Setup time =? Dont care Uncertainty=0.2 nsIn waveform window uncertainty value showing with negative sign(-0.2 ns). It means that the value is positive so it should be subtracted from the clock arrival time as per the calculation. Of course the same reflected in report window(22.982-0.2=22.782). Especially in incr coloumm the uncertainty value displayed with negative sign. For setup time also. value showing with negative sign(-0.021ns) in waveform window. Here I dont think the negative sign indicate it is negative setup for adding with clock arrival time. If so then why uncertainty value subtracted from clock arrival time? Here my question is very straight forward !!! how one value is subtracting from clock arrival time and another value is adding with clock arrival time when both values are in same sign(negative)? As per the equation when both values(setup&uncertainty) are in same sign (negative) then definitely both values should be added with clock arrival time! Am I making sense?