Low-Tech-Lösung
„A low-tech but effective diagnostic is to listen to the NIST WWV broadcast over the telephone while watching the clock in the Date & Time Control Panel.
You can listen to WWV over the telephone by calling 303-499-7111. (Long distance charges to Boulder, Colorado may apply.) [Please note that other time-of-day telephone services are often not accurate. If possible, use an ordinary ‚land line‘ telephone—digital cordless and cellular telephones introduce noticeable time delays. Telephone calls from outside the contiguous United States may go by satellite, which also introduces noticeable time delays.]
Differences of less than half a second are typical; differences up to one second are common.“
NIST Physics Laboratory: Configuring Apple Macintoshes to use NIST Time Servers, Section 7: Troubleshooting Hints