“Late; shortly before an anticipated event. Matthew’s parable of the laborers in the vineyard (20: 2-16) has the men hired at the eleventh hour being paid as much as the ones hired early in the morning, even though the eleventh-hour people only worked for an hour. From this sense of being barely in time to receive some benefit comes the concept of time running out.”

From “The Dictionary of Cliches” by James Rogers (Ballantine Books, New York, 1985).

And about the eleventh hour he went out, and found others standing idle, and saith unto them, Why stand ye here all the day idle? (Matthew 20:6)

They counted 12 hours from 6 a.m. (or the hour of sunrise) to 6 p.m (or the hour of sunset). Thus the third hour corresponds to our our 9 a.m., the sixth to 12 o’clock, or noon, the ninth to 3 p.m., and “the eleventh hour” to 5 p.m.