ATTINY As A Clock?

Will using time library with an Attiny/45/85/84 ( http://playground.arduino.cc/Code/Time ) create an accurate clock? I keep on seeing people using these external crystal things. Do I need one? Isn't millis pretty dang accurate? How would I sync the clock with real time?

Depending on where you live check for clock modules that use the time signal that is transmitted wirelessly.

EU, Japan and if I am not mistaken the US too offer these signals.
If that is no option a module that gets the time from the GSP satellites might be an option.
If quartz accuracy is enough and you want an external time keeper a RTC module from a PC might work pretty good and it will keep the time without external power supply through tiny batteries if necessary.