The time constant is a specification used to characterize a response to a step change in a first order linear time-invariant system. Therefore, a "Thermal Response Time Constant" is a specification used to characterize the response to a step change in temperature of a system that is modeled to have a first order linear time-invariant temperature characteristic. This means that the DS1922L takes 130 seconds to reach about 63.2% of the final temperature. By using the natural exponential decay function, it takes about 2.30259 time constants to reach 90% of the final temperature. 130 seconds times 2.30259 is equal to 299.227 seconds - this is the value reported in Note 17.