If I hear someone say "that clock is five minutes fast", I assume it's five minutes ahead but reasonably accurate in tracking the passage of time. This is in fact true for the clock in my hallway, which has been about five minutes ahead for the past year or so since it was last set, and I just haven't bothered to reset it correctly.
If someone meant that the clock actually ran faster than time does, I'd expect to hear something like "that clock runs a little fast" rather than "is five minutes fast".
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If someone meant that the clock actually ran faster than time does, I'd expect to hear something like "that clock runs a little fast" rather than "is five minutes fast".