This page describes how you can use an Atmel ATmega32U4 (e.g. an Arduino Leonardo, Teensy or clone) and a DCF77 receiver to build a (hot-pluggable) HID USB real-time (radio) clock (I’ve named it dcf77-hid-usb-rtc), for usage with Linux systems.
Because it is a radio clock, it will always have the exact time, at least as long as it will receive a signal from the German DCF77 time code transmitter in Mainflingen. So it is perfectly usable as a reliable stratum 0 time source for your network (if a resolution of one second fits your needs). As this isn’t a scientific paper about accurate and reliable real-time clocks, I will not go further into the details of time keeping. As a first step you might read the Wikipedia article about NTP.
It’s my second project in a small series about hot-pluggable USB RTCs. I’ve described my first project here: How to build an USB real-time clock (usb-rtc). A third one might follow.
The overall cost for one of those thingies I’m describing here is about 25€, which isn’t really cheap. But I find it a valuable thingy because the result is a hot pluggable RTC, usable by almost any device which has USB. So it’s very likely you will use it for much longer than the computer you currently want to build or search it for….