Before the coming of the railways (railroad) every town had its local time. With the introduction of Railway Time, time zones and Standard Time became the norm.
Local time is calculated by the line of longitude at which a town is placed. Longitude Zero (0° 0' 0") is the Greenwich Meridian and the line from which all lines of longitude are based.
The day is divided into 24 hours; If you measure local time each hour is 15° of longitude East or West of Greenwich (24 * 15° = 360°). Every 1° is 4 minutes of time (60 minutes of time / 15° of longitude) or 1 minute of time is 15' (minutes of longitude).
The co-ordinates at Oxford, England are: 51° 44' 60" North (of the Equator), 1° 15' 24" West (of Greenwich). So Oxford Time is 5 minutes and 2 seconds behind Greenwich Time.
At 9.05pm (9pm "Oxford Time") every evening Great Tom, Christ Church College's famous bell, rings out 101 times. This dates from the foundation of the college when the bell rang once for each of the college's original 101 students, in order to tell them to return to the college before the gates were locked. The bell then remains silent until 8am the next morning when it returns to striking every hour, on the hour (Greenwich Time) until 9pm in the evening.