silverbullet-icalendar/timezones.ts

// timezones.ts

/**
 * Mapping of Windows Timezone names to IANA Timezone names.
 * Sourced from Unicode CLDR data.
 */
export const WINDOWS_TO_IANA: Record<string, string> = {
  "Dateline Standard Time": "Etc/GMT+12",
  "UTC-11": "Etc/GMT+11",
  "Hawaiian Standard Time": "Pacific/Honolulu",
  "Alaskan Standard Time": "America/Anchorage",
  "Pacific Standard Time (Mexico)": "America/Santa_Isabel",
  "Pacific Standard Time": "America/Los_Angeles",
  "US Mountain Standard Time": "America/Phoenix",
  "Mountain Standard Time (Mexico)": "America/Chihuahua",
  "Mountain Standard Time": "America/Denver",
  "Central America Standard Time": "America/Guatemala",
  "Central Standard Time": "America/Chicago",
  "Central Standard Time (Mexico)": "America/Mexico_City",
  "Canada Central Standard Time": "America/Regina",
  "SA Pacific Standard Time": "America/Bogota",
  "Eastern Standard Time": "America/New_York",
  "US Eastern Standard Time": "America/Indiana/Indianapolis",
  "Venezuela Standard Time": "America/Caracas",
  "Paraguay Standard Time": "America/Asuncion",
  "Atlantic Standard Time": "America/Halifax",
  "Central Brazilian Standard Time": "America/Cuiaba",
  "SA Western Standard Time": "America/La_Paz",
  "Pacific SA Standard Time": "America/Santiago",
  "Newfoundland Standard Time": "America/St_Johns",
  "E. South America Standard Time": "America/Sao_Paulo",
  "Argentina Standard Time": "America/Buenos_Aires",
  "SA Eastern Standard Time": "America/Cayenne",
  "Greenland Standard Time": "America/Godthab",
  "Montevideo Standard Time": "America/Montevideo",
  "Bahia Standard Time": "America/Bahia",
  "Azores Standard Time": "Atlantic/Azores",
  "Cape Verde Standard Time": "Atlantic/Cape_Verde",
  "Morocco Standard Time": "Africa/Casablanca",
  "GMT Standard Time": "Europe/London",
  "Greenwich Standard Time": "Atlantic/Reykjavik",
  "W. Europe Standard Time": "Europe/Berlin",
  "Central Europe Standard Time": "Europe/Budapest",
  "Romance Standard Time": "Europe/Paris",
  "Central European Standard Time": "Europe/Warsaw",
  "W. Central Africa Standard Time": "Africa/Lagos",
  "Namibia Standard Time": "Africa/Windhoek",
  "Jordan Standard Time": "Asia/Amman",
  "GTB Standard Time": "Europe/Bucharest",
  "Middle East Standard Time": "Asia/Beirut",
  "Egypt Standard Time": "Africa/Cairo",
  "Syria Standard Time": "Asia/Damascus",
  "E. Europe Standard Time": "Europe/Chisinau",
  "South Africa Standard Time": "Africa/Johannesburg",
  "FLE Standard Time": "Europe/Kiev",
  "Turkey Standard Time": "Europe/Istanbul",
  "Israel Standard Time": "Asia/Jerusalem",
  "Kaliningrad Standard Time": "Europe/Kaliningrad",
  "Libya Standard Time": "Africa/Tripoli",
  "Arabic Standard Time": "Asia/Baghdad",
  "Arab Standard Time": "Asia/Riyadh",
  "Belarus Standard Time": "Europe/Minsk",
  "Russian Standard Time": "Europe/Moscow",
  "E. Africa Standard Time": "Africa/Nairobi",
  "Iran Standard Time": "Asia/Tehran",
  "Arabian Standard Time": "Asia/Dubai",
  "Azerbaijan Standard Time": "Asia/Baku",
  "Russia Time Zone 3": "Europe/Samara",
  "Mauritius Standard Time": "Indian/Mauritius",
  "Georgian Standard Time": "Asia/Tbilisi",
  "Caucasus Standard Time": "Asia/Yerevan",
  "Afghanistan Standard Time": "Asia/Kabul",
  "West Asia Standard Time": "Asia/Tashkent",
  "Ekaterinburg Standard Time": "Asia/Yekaterinburg",
  "Pakistan Standard Time": "Asia/Karachi",
  "India Standard Time": "Asia/Kolkata",
  "Sri Lanka Standard Time": "Asia/Colombo",
  "Nepal Standard Time": "Asia/Kathmandu",
  "Central Asia Standard Time": "Asia/Almaty",
  "Bangladesh Standard Time": "Asia/Dhaka",
  "N. Central Asia Standard Time": "Asia/Novosibirsk",
  "Myanmar Standard Time": "Asia/Rangoon",
  "SE Asia Standard Time": "Asia/Bangkok",
  "North Asia Standard Time": "Asia/Krasnoyarsk",
  "China Standard Time": "Asia/Shanghai",
  "North Asia East Standard Time": "Asia/Irkutsk",
  "Singapore Standard Time": "Asia/Singapore",
  "W. Australia Standard Time": "Australia/Perth",
  "Taipei Standard Time": "Asia/Taipei",
  "Ulaanbaatar Standard Time": "Asia/Ulaanbaatar",
  "Tokyo Standard Time": "Asia/Tokyo",
  "Korea Standard Time": "Asia/Seoul",
  "Yakutsk Standard Time": "Asia/Yakutsk",
  "Cen. Australia Standard Time": "Australia/Adelaide",
  "AUS Central Standard Time": "Australia/Darwin",
  "E. Australia Standard Time": "Australia/Brisbane",
  "AUS Eastern Standard Time": "Australia/Sydney",
  "West Pacific Standard Time": "Pacific/Port_Moresby",
  "Tasmania Standard Time": "Australia/Hobart",
  "Magadan Standard Time": "Asia/Magadan",
  "Vladivostok Standard Time": "Asia/Vladivostok",
  "Russia Time Zone 10": "Asia/Srednekolymsk",
  "Central Pacific Standard Time": "Pacific/Guadalcanal",
  "Russia Time Zone 11": "Asia/Anadyr",
  "New Zealand Standard Time": "Pacific/Auckland",
  "Fiji Standard Time": "Pacific/Fiji",
  "Tonga Standard Time": "Pacific/Tongatapu",
  "Samoa Standard Time": "Pacific/Apia",
  "Line Islands Standard Time": "Pacific/Kiritimati"
};

/**
 * Resolves an IANA timezone name from a given TZID string.
 * Supports Windows timezone names, direct IANA names, and UTC.
 */
export function resolveIanaName(tzid: string): string | null {
  if (!tzid || tzid === "UTC" || tzid === "None") return "UTC";
  
  // Heuristic: IANA names typically include a forward slash
  if (tzid.includes("/")) return tzid;
  
  return WINDOWS_TO_IANA[tzid] ?? null;
}

/**
 * Returns the UTC offset in milliseconds for a given IANA timezone at a
 * specific point in time. Positive = ahead of UTC, negative = behind UTC.
 * e.g. "America/New_York" in summer -> -14400000 (-4h)
 */
export function getUtcOffsetMs(ianaName: string, atDate: Date): number {
  // Trick: format the same instant in UTC and in the target zone,
  // parse both, and subtract.
  // "en-CA" produces "YYYY-MM-DD, HH:MM:SS" (unambiguous)
  const options: Intl.DateTimeFormatOptions = {
    timeZone: "UTC",
    hour12: false,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    second: "2-digit",
  };

  const utcFormatter = new Intl.DateTimeFormat("en-CA", options);
  const localFormatter = new Intl.DateTimeFormat("en-CA", { ...options, timeZone: ianaName });

  const formatToIso = (formatter: Intl.DateTimeFormat, date: Date) => {
    return formatter.format(date).replace(", ", "T");
  };

  const utcStr = formatToIso(utcFormatter, atDate);
  const localStr = formatToIso(localFormatter, atDate);

  const utcMs = new Date(utcStr + "Z").getTime();
  const localMs = new Date(localStr + "Z").getTime();

  return localMs - utcMs;
}