jtc-utils/src/util/getTimeZoneOffset.ts

Utilities for Japanese Traditional Companies

import { getTimeZone } from "./getTimeZone.ts";

export function getTimeZoneOffset(date: Date, tz: string) {
  const tzDate = relativeTime(date, tz);
  const zDate = relativeTime(date, getTimeZone());
  return zDate.getTime() - tzDate.getTime();
}

function relativeTime(date: Date, timeZone: string): Date {
  const parts: {
    year?: string;
    month?: string;
    day?: string;
    hour?: string;
    minute?: string;
    second?: string;
  } = {};

  const format = new Intl.DateTimeFormat("en-US", {
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    second: "2-digit",
    timeZone,
    hourCycle: "h23",
  });
  for (const part of format.formatToParts(date)) {
    parts[part.type as keyof typeof parts] = part.value;
  }
  return new Date(
    `${parts.year}-${parts.month}-${parts.day}T${parts.hour}:${parts.minute}:${parts.second}Z`,
  );
}