dateToIsoWithTz.js

dateToIsoWithTz.js

function dateToIsoWithTz(date, timeZone) {
  // Use 'sv-SE' locale because it's similar to ISO.
  const almostIsoFormatter = new Intl.DateTimeFormat('sv-SE', {
    timeZone,
    year: 'numeric',
    month: '2-digit',
    day: '2-digit',
    hour: '2-digit',
    minute: '2-digit',
    second: '2-digit',
    timeZoneName: 'short',
    hour12: false
  });

  const [isoDate, isoTime, isoTzOffset] = almostIsoFormatter.format(date).split(' ');

  // 'sv-SE' with timeZoneName: 'short' needs to be changed to ISO.
  // Examples:
  // GMT-7 => -07:00
  // GMT-10 => -10:00
  // GMT+5:30 => +05:30
  const fullTzOffset = isoTzOffset.replace(/(UTC|GMT)(.?)(.*)/, (_, prefix, sign, shortTime) => {
    // UTC or GMT with nothing after means no offset, i.e. ISO 'Z'.
    if (!sign) {
      return 'Z';
    }
    // Add leading and trailing zeros making sure to handle half-hour time zones.
    const fullTzTime = shortTime.includes(':') ? shortTime.padStart(5, '0') : `${shortTime.padStart(2, '0')}:00`;
    // 'sv-SE' uses char code 8722 for the minus sign instead of the usual char code 45.
    return (sign === '+') ? `+${fullTzTime}` : `-${fullTzTime}`;
  });
  
  return [isoDate, isoTime, fullTzOffset];
}