Method Deck / timezones

Convert a time between fixed UTC offsets

Converts a wall-clock time and date from one fixed UTC offset to another, handling the date rollover if any.

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Inputs

• date (string, required) : ISO 8601 date of the source time, YYYY-MM-DD
• time (string, required) : Source wall-clock time, HH:MM or HH:MM:SS (24-hour)
• from_offset (string, required) : Fixed UTC offset of the source time, e.g. -05:00
• to_offset (string, required) : Fixed UTC offset to convert into, e.g. +01:00

Outputs

• result_date: The resulting date in the target offset
• result_time: The resulting wall-clock time in the target offset, HH:MM:SS
• day_shift: 0 if the result is the same calendar date, +1 if it rolled to the next date, -1 if it rolled to the previous date

Steps

1. Parse date and time into a day-number (Hinnant days_from_civil, cited above) and seconds-since-midnight.
2. Parse from_offset and to_offset as signed HH:MM offsets from UTC, in seconds.
3. utc_seconds_total = day_number * 86400 + local_seconds - from_offset_seconds.
4. target_seconds_total = utc_seconds_total + to_offset_seconds.
5. result_day_number = floor(target_seconds_total / 86400); seconds_into_day = target_seconds_total - result_day_number * 86400.
6. Convert result_day_number back to a civil date (civil_from_days) and seconds_into_day back to HH:MM:SS.
7. day_shift = result_day_number - original_day_number.

Constants

None.

Sources

• chrono-Compatible Low-Level Date Algorithms (Howard Hinnant, 2026-09-30T19:41:35Z) - day-number kernel used to handle the date rollover; no time zone database is used anywhere in this method

Worked example and test vectors

Published card

• Address: 714d6ac92cd24a7700c5b04725a483ff
• Made at: 2026-09-30T22:39:30.722Z
• Method sha256: 24e49d17c9d4dfcb419885620c4095d0fce5d0a065f281d541b3d3d38f7cd2ab