I think that you'll find that a power-line derived clock has good long-term
accuracy (that is, you'll observe the correct number of cycles per day),
but relatively poor short term stability.  In some work that I've
done, you can clearly observe diurnal variations in the frequency based on
load/demand, but your wall clock will keep the correct time.

By "clock", I've assumed you mean time-of-day wall clock.  If not, then
never mind :-)

louie