How to calculate time from JSON knowing difference from UTC?

Question

JavaScript

Hello!
I decided to make a quick guide to countries here and collected JSON. The data in it looks like this:

...
{
    "country": "Sri Lanka",
    "phoneCode": "+94",
    "capital": "Colombo",
    "abbr": "LKA",
    "population": "21413249",
    "code": "LK",
    "currency": "LKR",
    "timezones": "UTC+05:30"
  }
...

I output them with a script:

const outputHtml = (fits) => {
  if (fits.length > 0) {
    const html = fits
      .map(
        (fit) => `
          <div class="card">
            <div class="card-top">
              <div class="card-title">${fit.country}</div>
              <img src="./img/flags/${fit.code}.svg">
            </div>
            <div class="card-data">
              <div>Population: ${fit.population.replace(/\B(?=(\d{3})+(?!\d))/g, ' ')}</div>
              <div class="capitalCity">Capital: <span class="capitalName">${fit.capital}</span>
                <div id="capitalWeather"></div>
              </div>
              <div>Country Code: ${fit.phoneCode}</div>
              <div class="currency">Сurrency: <span>${fit.currency}</span>
                <div id="currencyData"></div>
              </div>
              <div>Time: ${fit.timezones}</div>
            </div>
          </div>
        `
      )
      .join('');
    document.getElementById('countryList').innerHTML = html;
  }
};

Question: how to calculate the current time in different cities, knowing your time in UTC and the difference in the city's time from UTC (including the difference in minutes)?
As a result, now it looks like this, but every time it’s probably not very convenient to recalculate the time in your mind:

Answer 1

play around with possible timezones

function offsetToMs(s) {
  const [h, m] = s.replace(/utc/i, '').split(':').map(x => Number(x.replace(/\D/, '')))
  const ms = (h * 60 * 60 * 1000) + (m * 60 * 1000)
  return s.includes('-') ? -ms : ms
}

off = offsetToMs("UTC+05:30")
console.log( new Date(new Date().getTime() + off) )

UPD
ok, this just adds an offset to the browser time, but if "knowing your time in UTC", then you can fix it