Child of a Metal God

I'd internalized this without thinking of it: I say 'ahead' when the clock's set 5 minutes fast but is otherwise reliable, because I've spent a lot of time messing about inside vintage mechanical wristwatches. The mainspring attaches to a central pivot, which has an arm on it, allowing it to be varied. That changes the spring tension, which varies the watch speed. The arm end has a pointer on an inscribed arc, that's labeled "fast" and "slow" on the two ends.
Thing is: until you pointed this out, I had never consciously thought about my usage of ahead/behind vs fast/slow, so I must have been interpolating what other people were saying, unconsciously.