Correct and a great explanation Kevin.
In the case of DRGW 315, I do know that it's time (15 years) will run out on May 6, 2022. The clock started one year after the first flu was installed on May 6, 2006. I doubt that it will reach 1472 Service Days (as defined by Kevin) due to limited use.
As Kevin explained, most operations will reach the 15 years before 1472 service days are accumulated. A notable exception is the D&S as some of their locomotives have reached 1472 service days prior to 15 years.
They use em!