Why London is the global reference zone

Until 1884, every country used local solar time. Then the International Meridian Conference — held in Washington DC — settled on Greenwich as the 0° meridian. It was partly because British Admiralty maps had already standardized on it, and partly because the United States was already running its railway zones from Greenwich. The legacy is that UTC offsets are all measured from this city. GMT and UTC differ by a fraction of a second due to leap-second corrections, but for scheduling they're identical.

The City of London business day

The Square Mile has short, intense hours. Traders arrive by 7am for market prep, the London Stock Exchange opens at 8am GMT (9am BST in summer), and the main overlap with New York runs roughly 1pm to 4pm GMT, when both cities are simultaneously active. Lunches are quick or at desks. Most of corporate London has cleared out by 6pm, which is why evening calls to New York usually get scheduled on US time, not UK time — London is already off the clock.

BST — the British daylight saving headache

Britain switches to BST on the last Sunday of March and back to GMT on the last Sunday of October. Parliament debates removing DST every few years and never does. The really painful bit is the March transition: the US springs forward a week or two earlier than the UK, so for those two weeks the usual 5-hour London ↔ New York gap shrinks to 4. For exactly 14 days, everyone's recurring weekly call happens an hour earlier in one time zone or the other, and nobody remembers why.

This single mismatch breaks more recurring calls every year than anything else in global scheduling. Flag it in advance, warn the team, and update the invites. Or use a tool that handles it automatically.