Stratux sells a u-blox 8 USB dongle that I've had decent luck with for this timekeeping purpose, although it can be tricky to get a clear signal indoors, and I had to bootstrap the module's first connection outside with usb-otg on a smartphone. But pretty acceptable results for under $20 when feeding it into ntpd. Not exactly plug and play to get it to work, though. But pretty reasonable for most of the diy crowd around here.

I need to tape over the green led due to photosensitivity, though. It's shockingly bright.