Your 12 MHz oscillator will have some frequency error depending on crystal quality, oscillator circuit design, temperature, aging, etc. That frequency error will cause an equivalent speed-up or slow-down of your clock (if you don't apply some sort of software correction). For example, if your oscillator is actually 12.00012 MHz (that's 0.001% too high, or +10 ppm) then your clock will run 0.001% too fast. Since one day hasan 86400 seconds, your clock would gain 0.864 seconds per day.
Or maybe you knew all that basic stuff, and were wondering how to compute the actual frequency error of your specific board? Your microprocessor data sheet may say something like this, "if you use this brand crystal and this circuit layout, then the frequency accuracy will be (whatever)".