Time zones are regions of the Earth that observe a uniform standard time for legal, commercial, and social purposes. The world is divided into 24 primary time zones, each roughly 15 degrees of longitude wide, corresponding to one hour of time difference. The primary reference point is Coordinated Universal Time (UTC), formerly known as Greenwich Mean Time (GMT), which runs through the Royal Observatory in Greenwich, London.

Time zones to the east of UTC are ahead (positive offset), while zones to the west are behind (negative offset). For example, Tokyo (UTC+9) is 9 hours ahead of UTC, while New York (UTC-5 during standard time) is 5 hours behind. Some regions use fractional offsets, such as India (UTC+5:30) and Nepal (UTC+5:45).

Daylight Saving Time (DST) is the practice of advancing clocks during warmer months so that evenings have more daylight. Not all countries or regions observe DST, and those that do may change their clocks on different dates. This can cause temporary shifts in the time difference between regions.

For example, when the US observes DST (March to November), New York shifts from UTC-5 (EST) to UTC-4 (EDT). Meanwhile, countries like Japan, India, and most of Africa do not observe DST at all. The Southern Hemisphere observes DST during opposite months, adding another layer of complexity to international time coordination.