getGmtOffset.ts

export const getGmtDiff = (timeZone: typeof IANNA_TIME_ZONES[number]["key"]) => {
  const date = new Date();
  const GMT_TIME_UTC_DATE = date.getUTCDate();
  const GMT_TIME_HOURS = date.getUTCHours();
  const GMT_TIME_MIN = date.getUTCMinutes();

  const timeZoneDate = new Date(new Date().toLocaleString("en-US", { timeZone }));

  const h = timeZoneDate.getHours();

  // NOTE: Remember that adding a negative number subtracts
  let dateLineOffset = 0;
  if (timeZoneDate.getDate() < GMT_TIME_UTC_DATE) dateLineOffset = -24;
  if (timeZoneDate.getDate() > GMT_TIME_UTC_DATE) dateLineOffset = 24;

  const hrs = h - GMT_TIME_HOURS + dateLineOffset;

  let plusOrMinus = " ";
  if (hrs > 0) plusOrMinus = "+";
  if (hrs < 0) plusOrMinus = "-";

  // NOTE: Remember that adding a negative number subtracts
  const minDiff = timeZoneDate.getMinutes() - GMT_TIME_MIN;
  const hrsAccountingMinutes = minDiff < 0 ? hrs + minutes(minDiff).in("hours") : hrs;

  // NOTE: Remember that adding a negative number subtracts
  const finalHours = String(Math.abs(Math.floor(hrsAccountingMinutes))).padStart(2, "0");
  const finalMinutes = String(minDiff < 0 ? 60 + minDiff : minDiff).padStart(2, "0");

  return `(GMT${plusOrMinus}${finalHours}:${finalMinutes})`;
};