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.
The Tom Tower at Christ Church in Oxford was built by Sir Christopher Wren. Christ Church was founded in 1524 by Cardinal Wolsey as Cardinal College and refounded as Christ Church by Henry VIII in 1546 on his fall from grace. Tom Tower, above the main gate, was designed by Sir Christopher Wren in 1681 and houses the bell Great Tom. The college chapel is also Oxford's cathedral, the smallest in England.
In 1525, Henry VIII's grandiose cardinal chief minister, Thomas Wolsey, established a college on the present site. The hallmarks of Wolsey, ambition and magnificence, are imprinted on both of his great building projects, Cardinal College as much as Hampton Court. His college was planned on a scale not even imagined before in Oxford. Its hall was intended to be the most splendid in the university and the Great Quadrangle which he planned at its heart (now Christ Church's Tom Quad) remains the largest quadrangle in Oxford.
Christ Church occupies a unique position among the colleges of Oxford University, not only because it is the largest college in England's oldest University, nor simply because it has played a central - at times pre-eminent - role in Oxford's history. Christ Church's unique dual role as a college of the university and the cathedral of the diocese of Oxford gives it an unparalleled special status.
Famous past students that attended Christ Church include the founder of the Methodist church, John Wesley, Alice in Wonderland author C L Dodgson (alias Lewis Carroll), Albert Einstein, Auberon Waugh and W. H. Auden. William Penn, founder of Pennsylvania also studied here, and over a dozen British Prime Ministers including Lord Liverpool, George Canning, Sir Robert Peel, William Gladstone, Anthony Eden and Sir Alec Douglas-Home.