Profile: Christ Church Cathedral, Oxford
The Bishop of Oxford's seat is unique in more ways than one.
Perhaps most striking for the visitor is Christ Church Cathedral's approach to time-keeping. While a service of choral evensong at another cathedral might begin at 6pm, at Oxford they'll wait until five past before the choristers file in.
They're not late, of course, merely keeping 'Oxford Time', which the cathedral still observes and apparently runs five minutes behind GMT/BST.
16 He sent from above, he took me, he drew me out of many waters. 17 He delivered me from my strong enemy, and from them which hated me: for they were too strong for me. 18 They prevented me in the day of my calamity: but the Lord was my stay. 19 He brought me forth also into a large place; he delivered me, because he delighted in me.
- Psalm 18, KJV, used in Evensong the night I visited Christ Church
The Victorians brought their zeal for measuring and standardising to the practice of time-keeping. Previously, people had generally taken the local time from the position of the sun, which was fine if you only had appointments to keep within your own town or village. Nobody was travelling or communicating by means faster than a man on horseback.
But the Industrial Revolution required something more precise. With increasing industrialisation came railways, and suddenly the several minutes' difference in local time between, say, Norwich and Bristol made keeping a national train timetable impossible (some might unkindly say this is still the case).
From the 1850s, most British towns were setting their clocks to 'Railway time', determined by the position of the sun over the Greenwich Observatory in London, and by 1880 this Greenwich Mean Time was adopted as a nationwide standard. 
Most towns, except parts of Oxford, that is. Because of its distance to the west of the Greenwich Meridian, Oxford was always around five minutes behind GMT, and Christ Church has seen no reason to ever catch up. Somewhere like Oxford, after all, doesn't get to be so prestigious by ignoring traditions. So the cathedral, uniquely, has its own little unofficial time zone (it seems even the other colleges follow GMT).
But the unique bit that's probably more interesting is Christ Church's dual function as Church of England cathedral and chapel of Christ Church College at Oxford University. The building's role for both the diocese and university came about as a result of the power struggles and religious upheaval of the early 1500s.
The Doomsday Book mentions canons based around the site as early as 1086, and a monastery was re-established here in 1122. Before the 1500s, an Augustinian priory held the relics of St Frideswide, which attracted a small stream of pilgrims, including some kings. Frideswide, Oxford's patron saint, was a local Anglo-Saxon princess who refused to marry Anglo-Saxon Prince Aelfgar or Algar, even when he tried to take her by force, and chose life in a nunnery instead. Her remains are still held in a shrine in a corner of the cathedral. 
However, it was Cardinal Thomas Wolsey who lifted the site out of relative obscurity when, around 1524, he received papal permission to dissolve the priory and establish instead a new college for the university. He'd been educated at Oxford himself, so founding new colleges was a way of giving back, and also showing how far he'd come and how powerful he now was. He used funds from various other dissolved monasteries, and demolished some of the priory building to make way for the Great Quadrangle at his new Cardinal College.
Unfortunately for Cardinal Wolsey, his position in Tudor England, as with so many servants of Henry VIII, was dependent on that king's favour, which Wolsey lost in 1529. Henry's wife, Catherine of Aragon, had visited St Frideswide's shrine to pray for a son, to no avail, and it seems the saint was no more inclined to help Wolsey in his quest to have Catherine's marriage annulled so that Henry might try another wife. Frustrated with the lack of progress, Henry cast off Wolsey, who died on the way to his trial for treason (ie. displeasing the king), and took over the endowment of Cardinal College (renamed King Henry VIII College).
Henry didn't decide to follow Wolsey's rebuilding plans, so much of the building that became his new cathedral in Oxford was retained from the earlier priory. In 1542, Henry created a new episcopal see cut from the unwieldy and vast see of Lincoln, and in 1546 established the renamed Christ Church, Oxford as the cathedral and bishop's seat. The following year, Christ Church was again left dangling when Henry himself died, along with any further plans he may have had. Since then, the college and cathedral have maintained a unique and idiosyncratic existence. Given the unusual arrangement behind the founding of Christ Church and the diocese, it's no surprise that they keep to their own time zone. 
There have been some changes since Henry VIII's death, most notably the huge Tom Tower that now marks the entrance to the Quad from the city. This bell tower was designed by Christopher Wren, presumably when he had a day off from rebuilding all the churches in the City of London (the building work was done by his protégé Christopher Kempster).
A little before Wren, Christ Church's big contribution to British history was in becoming the seat of Charles I's government when he left London and raised an army to fight parliament. The Christ Church Deanery was his base until his eventual defeat, capture and execution in 1649. Ever since, Christ Church has been a supporter of the English establishment, and has produced more Prime Ministers than any other college at Oxford or Cambridge (accounts vary, but perhaps as many as 16 at the time of writing). 
Finally, it would be remiss of me not to mention a former Dean of the cathedral, Henry Liddell, for whose children, including daughter Alice, Lewis Carroll is supposed to have written his famous children's books. Carroll (Charles Lutwidge Dodgson) was a mathematics tutor at Christ Church, while Liddell was Dean at the cathedral, and although the literal basis for Alice may be imaginary, this hasn't stopped an Alice memorabilia and tourist culture from exploiting the connection.
 https://www.chch.ox.ac.uk/cathedral[accessed 08/03/20]
 http://www.bbc.co.uk/history/british/victorians/speed_01.shtml[accessed 08/03/20]
 http://www.berkshirehistory.com/legends/frideswide02.html[accessed 08/03/20]
 https://www.british-history.ac.uk/vch/oxon/vol2/pp97-101[accessed 08/03/20]
 https://www.chch.ox.ac.uk/sites/default/files/Visitor_Information-gb.pdf[accessed 08/03/20]
 https://www.britannica.com/biography/Lewis-Carroll[accessed 08/03/20]