I was really surprised to hear that one second in a million years is only just sufficient for the accurate functioning of GPS. But if you think about it light covers a distance of 300,000,000 metres in one second, which would represent a drift of 300 metres a year or nearly a metre a day for the accuracy of GPS.
So in order to maintain positional accuracy to within one metre and a clock accurate to one second in a million years, all the GPS satellites would need to be synchronised once a day.
A clock that can measure time to within 1 second in ~15 billion years represents an accuracy of 2 cm a year. This is is not very much slower than continental drift and is equivalent one wavelength of green light every 3 seconds.
I don't think clocks accurate to 1 second in 15 billion years will be required for day-to-day use like GPS for the time being, but I bet you a pint of frothy nut brown ale that someone will find some use for a clock that accurate some time soon. I wouldn't be surprised if that person is a cosmologist.
