tz-offset-calc - casidoo

tz-offset-calc.js

let humans = [
  { name: 'Clara', timezone: 'UTC−4:00' },
  { name: 'Cami', timezone: 'UTC−7:00' },
  { name: 'Ximena', timezone: 'UTC−5:00' },
];

/**
 * Return Hour & Minutes of a given human
 * Optional 12/24hs format with a boolean
 **/
function localTime(personName, hour12 = true) {
  if (typeof personName !== 'string') throw new Error('Invalid name');
  const person = humans.find((el) => el.name === personName);
  if (!person) throw new Error('Person not found');

  const tz = person.timezone;
  const tzOffset = Number(
    tz.replace('UTC', '').replace(':', '.').replace('−', '-')
  );
  if (typeof tzOffset !== 'number') throw new Error('Invalid Timezone');

  const current = new Date();
  const localDiffToUTC = new Date().getTimezoneOffset() * 60000;
  // Needed to account for the local tz of the browser running the function
  const personDiffToUTC = tzOffset * 3600000;
  const personDiffToLocal = localDiffToUTC + personDiffToUTC;
  const personDate = new Date(current.getTime() + personDiffToLocal);

  return personDate.toLocaleString('en-US', {
    hour: 'numeric',
    minute: 'numeric',
    hour12,
  });
}

localTime('Clara');