The core mechanic
Every timezone has a fixed offset from UTC (Coordinated Universal Time) — IST is UTC+5:30, JST is UTC+9:00, EST is UTC−5:00, and so on. Converting a time from one zone to another is simply computing the difference between the two zones' offsets and adding or subtracting that difference from the original time.
A worked example — same day
14:30 in IST (UTC+5:30) converts to 04:00 in EST (UTC−5:00) — a 10.5-hour difference subtracted from the original time, and both times fall on the same calendar day since the result doesn't cross midnight.
A worked example — the date actually changes
23:30 in IST converts to 03:00 in JST (UTC+9:00, 3.5 hours ahead of IST) — but adding 3.5 hours to 23:30 pushes past midnight, so the result is reported as "03:00, +1 day": the converted time is on the calendar day AFTER the original time's date, not the same day. Ignoring this date shift and just reading "03:00" without the "+1 day" note would silently misplace the converted time by a full 24 hours.
Why a date shift is easy to miss and important to get right
Scheduling a call, a flight, or a deadline across timezones without tracking the date shift is a common real source of confusion — "let's meet at 9pm my time" can land on a completely different calendar date for someone in a timezone far enough ahead or behind, and missing that shift is exactly the kind of error that causes someone to show up a day early or late. Any timezone conversion tool needs to report the date relationship explicitly (same day, +1 day, or −1 day), not just the converted clock time alone.
Seeing several zones at once
A world clock applies this identical conversion logic to several timezones simultaneously from one reference time — 14:30 IST converts to 09:00 UTC, 04:00 EST, 01:00 PST, 10:00 CET, 18:00 JST, and 19:00 AEST all at once, each computed independently using the same fixed-offset arithmetic against the reference zone. This is useful for anyone coordinating across multiple regions at once, rather than converting one pair of zones at a time.
A limitation worth knowing
This fixed-offset model doesn't account for daylight saving time (DST), which shifts some regions' effective UTC offset by an hour during part of the year — a timezone using a fixed offset gives the correct conversion for that offset, but the real-world offset for a DST-observing region can differ by an hour depending on the time of year. For contexts where DST matters (scheduling something months in advance across a DST-observing region), double-check the specific date's actual offset rather than relying solely on a fixed-zone conversion.