Time Zone Converter Calculator
Converts a clock time from one UTC offset to another, with optional daylight saving adjustment. Use it when scheduling international meetings, coordinating remote teams, or planning cross-timezone travel.
About this calculator
Every time zone is defined by its offset from Coordinated Universal Time (UTC). To convert a source time to a target time zone, the formula is: targetHour = (sourceHour − sourceOffset + targetOffset + dstAdjustment + 24) mod 24. First, sourceHour − sourceOffset converts the local time to UTC. Adding targetOffset then shifts to the target zone. The +24 before the modulo prevents negative intermediate values, and mod 24 wraps the result back into the 0–23 hour range. The optional dstAdjustment (typically +1 or 0) accounts for Daylight Saving Time in either the source or target zone. All hours are in 24-hour format.
How to use
Convert 14:00 (2 PM) New York time (UTC−5, DST active so effective offset = −4) to Tokyo time (UTC+9, no DST). Enter sourceHour = 14, sourceOffset = −4 (accounting for DST), targetOffset = 9, dstAdjustment = 0. Calculation: (14 − (−4) + 9 + 0 + 24) mod 24 = (14 + 4 + 9 + 24) mod 24 = 51 mod 24 = 3. So 2 PM in New York (EDT) is 03:00 the next day in Tokyo — a 13-hour difference.
Frequently asked questions
How do I handle Daylight Saving Time when converting between time zones?
Daylight Saving Time shifts a zone's effective UTC offset by +1 hour during summer months. When DST is active in the source zone, increase the source offset by 1 (e.g., EST UTC−5 becomes EDT UTC−4). When DST is active in the target zone, increase the target offset by 1. The calculator's dstAdjustment field lets you apply a net +1 or −1 correction if needed. Note that DST dates differ by country: the US switches in March/November while Europe switches in late March/October, creating brief periods where the offset between two zones is temporarily different.
What is the difference between UTC offset and a named time zone?
A UTC offset (e.g., +5:30) is a fixed numerical shift from Universal Time, while a named time zone (e.g., 'India Standard Time') is a political/geographic designation that includes rules for DST and historical changes. This calculator works with numeric UTC offsets, which are precise and unambiguous. Named zones can change due to government decisions — countries occasionally shift their time zone permanently. For long-term scheduling systems, it is safer to store events in UTC and convert using a time-zone database like IANA (used by most programming languages).
Why does the formula add 24 before taking the modulo when converting time zones?
When you subtract a large positive source offset from an early morning hour, the intermediate value can become negative (e.g., 2 − 5 = −3). Negative values fed into a modulo operation behave unexpectedly in many programming environments: −3 mod 24 might return −3 instead of 21. Adding 24 before the mod ensures the value stays positive for all realistic inputs where the offset difference does not exceed 24 hours, which is always true since UTC offsets range from −12 to +14. The result is then correctly wrapped into the 0–23 range.