The time zone converter takes a time in one place and shows what it reads on the clock in another. The British end of it is less straightforward than it looks, because the UK does not sit on a single offset all year: it keeps Greenwich Mean Time in winter and moves to British Summer Time, one hour ahead, from the last Sunday in March to the last Sunday in October. Writing "GMT" for a July meeting is therefore wrong by an hour — the correct label in summer is BST, or UTC+1.
The gaps that catch people out are the ones where the other country changes its clocks on a different date. The United States moves in March and November, so for roughly two weeks in March and one in November the difference between London and New York is four hours rather than the usual five. Australia moves in the opposite direction, because its summer is the northern winter, and the gap between London and Sydney swings between nine and eleven hours across the year. A recurring call booked once and assumed permanent will drift out of position at each changeover.
Times here are shown on the 12-hour clock with lowercase am and pm alongside the 24-hour form, since British usage mixes the two freely and a schedule that reads 7.30 pm in one column and 19:30 in the next is entirely normal.
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