When the time in London is 1 a.m., the time in New York City is 8 p.m. the previous day and noon the same day in Sydney, Australia. Not all time zones differ by a multiple of an hour; when it's 1 a.m. in London, it's 6:30 a.m. the same day in New Delhi, India and 4:30 a.m. the same day in Tehran, Iran.
The difference in the local time between two cities may vary depending on the time of year, because daylight saving time, known as "summer time" in many regions of the world, is not used in some places, including Adelaide, Australia. During daylight saving time, the time in Adelaide is 15.5 hours ahead of New York City but the difference during standard time is 14.5 hours. Like most of Arizona, Phoenix does not use daylight saving time, so it is two hours behind New York City during daylight saving time and three hours behind during standard time.
Other oddities can affect time differences between world cities. For example, some places use a quarter-hour offset, such as Kathmandu, where it is five hours and 45 minutes later than in London. Australia has horizontal and vertical time zones during summer, leading to an often confusing array of time differences between cities. Even when two cities use daylight saving time, the times in the two cities may be offset by an hour at particular times of the year because they switch to daylight saving time on different dates. The date on which Brazil makes this switch changes from year to year, by government decree.