If you want to store a time for an appointment for a group of people all in different timezones—store a specific point in time:

{
    'title' : 'Meeting about getting rid of Daylight Saving Time',
    'time' : { 
        'ts': ISODate("2018-04-19 15:27:32"),
        'tz': 'Europe/London',
    },
    'attendees' : [ 'Arthur David Olson', 'Paul Eggert' ],
}

If you want to future proof a specific date and time (Oct 1st, 13:00)-ie, storing a time in local time:

{
    'title' : 'Giving a talk: Advanced Date/Time handling with PHP',
    'time' : { 
        'dt': '2018-04-18 19:40',
        'tz': 'Europe/London',
    }
}