famdone/timezone.ts

export const AUTO_TZ = "auto";
export const DEFAULT_TZ = "UTC";

export function resolveTz(tz?: string | null): string {
  if (tz && tz !== AUTO_TZ) return tz;
  try {
    return Intl.DateTimeFormat().resolvedOptions().timeZone || DEFAULT_TZ;
  } catch {
    return DEFAULT_TZ;
  }
}

export function dateStrInTz(date: Date, tz: string): string {
  const parts = new Intl.DateTimeFormat("en-US", {
    timeZone: tz,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
  }).formatToParts(date);
  const get = (type: string) =>
    parts.find((p) => p.type === type)?.value ?? "00";
  return `${get("year")}-${get("month")}-${get("day")}`;
}

export function weekdayInTz(date: Date, tz: string): number {
  const parts = new Intl.DateTimeFormat("en-US", {
    timeZone: tz,
    weekday: "short",
  }).formatToParts(date);
  const wd = parts.find((p) => p.type === "weekday")?.value ?? "";
  const map: Record<string, number> = {
    Sun: 0,
    Mon: 1,
    Tue: 2,
    Wed: 3,
    Thu: 4,
    Fri: 5,
    Sat: 6,
  };
  return map[wd] ?? date.getDay();
}

export function todayInTz(tz: string): string {
  return dateStrInTz(new Date(), tz);
}

export function addDaysStr(dateStr: string, days: number): string {
  const [y, m, d] = dateStr.split("-").map(Number);
  const dt = new Date(Date.UTC(y, m - 1, d + days));
  return dt.toISOString().slice(0, 10);
}

export function weekStart(payday: number, tz: string): string {
  const today = todayInTz(tz);
  const wd = weekdayInTz(new Date(), tz);
  const back = ((wd - payday) % 7 + 7) % 7;
  return addDaysStr(today, -back);
}

function monthStartStr(): string {
  const d = new Date();
  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}-01`;
}

function monthEndStr(month?: string): string {
  if (!month) {
    const d = new Date();
    month = `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}`;
  }
  const [y, m] = month.split("-").map(Number);
  const lastDay = new Date(y, m, 0).getDate();
  return `${month}-${String(lastDay).padStart(2, "0")}`;
}

export function periodStart(period: string, payday: number, tz: string): string {
  if (period === "daily") return todayInTz(tz);
  if (period === "weekly") return weekStart(payday, tz);
  if (period === "monthly") return monthStartStr();
  return weekStart(payday, tz);
}

export function periodEnd(period: string, start: string): string {
  if (period === "daily") return start;
  if (period === "weekly") return addDaysStr(start, 6);
  if (period === "monthly") return monthEndStr(start.slice(0, 7));
  return start;
}

export function wallClockToUtc(dateStr: string, time: string, tz: string): number {
  const [y, m, d] = dateStr.split("-").map(Number);
  const [h, min] = time.split(":").map(Number);
  let epoch = Date.UTC(y, m - 1, d, h, min);
  for (let i = 0; i < 4; i++) {
    const parts = new Intl.DateTimeFormat("en-US", {
      timeZone: tz,
      year: "numeric",
      month: "2-digit",
      day: "2-digit",
      hour: "2-digit",
      minute: "2-digit",
      hour12: false,
    }).formatToParts(new Date(epoch));
    const get = (type: string) =>
      parts.find((p) => p.type === type)?.value ?? "00";
    const gotDate = `${get("year")}-${get("month")}-${get("day")}`;
    let gotHour = parseInt(get("hour"), 10);
    if (gotHour === 24) gotHour = 0;
    const gotEpoch = Date.UTC(
      parseInt(get("year"), 10),
      parseInt(get("month"), 10) - 1,
      parseInt(get("day"), 10),
      gotHour,
      parseInt(get("minute"), 10),
    );
    if (gotEpoch === epoch && gotDate === dateStr) break;
    epoch = Date.UTC(y, m - 1, d, h, min) + (epoch - gotEpoch);
  }
  return epoch;
}

export const COMMON_TIMEZONES: string[] = [
  "Europe/London",
  "Europe/Paris",
  "Europe/Berlin",
  "Europe/Madrid",
  "Europe/Rome",
  "Europe/Amsterdam",
  "Europe/Lisbon",
  "Europe/Dublin",
  "Europe/Warsaw",
  "Europe/Stockholm",
  "Europe/Athens",
  "Europe/Istanbul",
  "Europe/Moscow",
  "America/New_York",
  "America/Chicago",
  "America/Denver",
  "America/Los_Angeles",
  "America/Toronto",
  "America/Vancouver",
  "America/Sao_Paulo",
  "America/Mexico_City",
  "Asia/Tokyo",
  "Asia/Shanghai",
  "Asia/Hong_Kong",
  "Asia/Singapore",
  "Asia/Seoul",
  "Asia/Kolkata",
  "Asia/Dubai",
  "Asia/Jerusalem",
  "Asia/Bangkok",
  "Asia/Manila",
  "Australia/Sydney",
  "Australia/Melbourne",
  "Australia/Brisbane",
  "Australia/Perth",
  "Australia/Adelaide",
  "Pacific/Auckland",
  "Africa/Johannesburg",
  "Africa/Cairo",
  "Africa/Lagos",
  "Africa/Nairobi",
  "UTC",
];