Line frequency in the USA  is actually very accurate over the long term - not with respect to atomic clocks - but the power companies do correct the line frequency.

In the suburb of Boston where we are located - you never have to correct an electrical clock between the daylight saving time changes. This of course assumes you haven't lost power - due to major snow storms etc....

If anyone is interested - this is a 1920's version of a line frequency monitor used in a power station - with a pendulum clock as the time standard.

http://electricclock.omeka.net/items/show/6

Thomas D. Erb
tde@electrictime.commailto:tde@electrictime.com /
Electric Time Company, Inc.
Office: 508-359-4396 x 117 / Fax: 508-359-4482
97 West Street Medfield, MA 02052 USA
www.electrictime.comhttp://www.electrictime.com
[Facebook]https://www.facebook.com/pages/Electric-Time-Company-Inc/127918073950854?ref=hl[Twitter]https://twitter.com/tower_clocks[pinterest]https://www.pinterest.com/electrictime/
[htmlsig.com]http://www.electrictime.com/

Amusing story:

Back in high school,  Ewa Beach Hawaii, I used to notice that the second hand on the clocks would sometimes slow significantly.  This turned out to be caused by the man who would later become my father in law.
He ran the steel mill, melting old cars into rebar, which was electrically fired, and would slow down the Kahe point generator when he started the melt.

I assume they dialed out those "bumps" through the day.