if you want to check the difference in a time between two dates, you can simply check if second timezone is lesser or greater from your first desired timezone and subtract or add a time.
const currTimezone = new Date().getTimezoneOffset(); // your timezone
const newDateTimezone = date.getTimezoneOffset(); // date with unknown timezone
if (currTimezone !== newDateTimezone) {
// and below you are checking if difference should be - or +. It depends on if unknown timezone is lesser or greater than yours
const newTimezone = (currTimezone - newDateTimezone) * (currTimezone > newDateTimezone ? 1 : -1);
date.setTime(date.getTime() + (newTimezone * 60 * 1000));
}