Perhaps "eating by the clock" has roots in European manor houses filled with servants, where the lady of the house got to choose the times of meals (within the narrow window of what was considered right and proper). In more modern times, eating by the clock has to do with factory lunch breaks and with school bells.
Don't be the clock's mother. Don't watch the clock to see if it's time to eat. Watch your child. Or watch the clock to see if it's time to offer another snack, but don't let the clock say "not yet" or "Must EAT!"
It isn't good parenting or self control for an adult who has reproduced to be looking to a mechanical device to make decisions for her. Clocks are great for meeting people at a certain time, but they were never intended to be an oracle by which mothers would decide whether to pay attention to a child or not. Your child knows whether he's hungry. You don't. The clock doesn't either, never did, and never will.
from page 163 of The Big Book of Unschooling (page 182 of newer editions)
photo by Sandra Dodd