Seriously? You care enough about temporal accuracy buy an atomic clock but you don't know how to build one? We won't tell. Thankfully, DIY Physics has a great tutorial on how to build your own with parts from eBay.
The core of the makeshift atomic clock is an Efratom Model M-100 Rubidium Frequency Standard oscillator originally designed for use in military communications and navigation. They can now apparently be found online in the $US150 to $US200 price range, and rely on the radioactive element Rubidium 87 to provide a constant stable 10MHz signal.
But turning it into the source signal for an atomic clock takes a little more know-how than building the Radio Shack electronics kits of yesteryear. Keeping it at the perfect temperature is critical for an accurate signal, and you'll need to supply 25 volts of DC power to fire it up.
So it's not quite an after school activity, but if you're a technically inclined helicopter pilot parent who likes to help -- aka do everything -- with your kids schoolwork, this is an A+ science fair project waiting to happen. [DIYPhysics via Make]