Method Deck / timezones

Meeting time across three fixed offsets

Converts a single UTC date and time into local wall-clock times at three fixed UTC offsets, noting any date shift.

Published for review. The Founder is checking every card himself; anything he strikes comes down the same hour.

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utc_date (string, required) : ISO 8601 UTC date of the meeting instant, YYYY-MM-DD
utc_time (string, required) : UTC wall-clock time of the meeting instant, HH:MM or HH:MM:SS
offset_a (string, required) : First fixed UTC offset, e.g. -05:00
offset_b (string, required) : Second fixed UTC offset, e.g. +01:00
offset_c (string, required) : Third fixed UTC offset, e.g. +09:00

Parse utc_date and utc_time into a day-number (Hinnant days_from_civil, cited above) and seconds-since-midnight, forming a single UTC instant in total seconds.
Parse each of offset_a, offset_b, offset_c as a signed HH:MM offset from UTC, in seconds.
For each offset: local_total_seconds = utc_total_seconds + offset_seconds; local_day_number = floor(local_total_seconds / 86400); seconds_into_day = local_total_seconds - local_day_number * 86400.
Convert local_day_number back to a civil date and seconds_into_day back to HH:MM:SS.
Compare local_day_number to the original UTC day-number to report same day, next day, or previous day.

None.

chrono-Compatible Low-Level Date Algorithms (Howard Hinnant, 2026-09-30T19:41:35Z) - day-number kernel used for the date-rollover arithmetic; no time zone database is used

Name	Inputs	Answer
worked example	`{"utc_date":"2026-09-30","utc_time":"18:30:00","offset_a":"-05:00","offset_b":"+01:00","offset_c":"+09:00"}`	-05:00: 2026-09-30 13:30:00 (same day); +01:00: 2026-09-30 19:30:00 (same day); +09:00: 2026-10-01 03:30:00 (next day)
test vector: all same day	`{"utc_date":"2026-01-01","utc_time":"12:00:00","offset_a":"+00:00","offset_b":"+02:00","offset_c":"-03:00"}`	+00:00: 2026-01-01 12:00:00 (same day); +02:00: 2026-01-01 14:00:00 (same day); -03:00: 2026-01-01 09:00:00 (same day)
test vector: previous day rollover	`{"utc_date":"2026-06-15","utc_time":"01:00:00","offset_a":"-08:00","offset_b":"+00:00","offset_c":"+05:30"}`	-08:00: 2026-06-14 17:00:00 (previous day); +00:00: 2026-06-15 01:00:00 (same day); +05:30: 2026-06-15 06:30:00 (same day)

Address: 7933c8f804bbcbd8484868d62edb0dd0
Made at: 2026-09-30T22:39:30.722Z
Method sha256: fa06cac422708aa2c0424edd41a7a1ab00f79c3eff2718cb74a4c0b153ff4976