GPS 1850W time too fast

[Comet]

I have a GPS 1850W and the display time is 18 seconds too fast. I have a Furuno GPS 170 and GPS 150, and the time on them is correct, but the 1850 is 18 seconds too fast. An idea as to why?

 

[C-Bass]

good afternoon Comet,

What is the source of GPS position and time for the GP1850w?