How to aggregate by year-month-day on a different timezone

Question

I have a MongoDB whom store the date objects in UTC. Well, I want to perform aggregation by year,month day in a different timezone (CET).

doing this, works fine for

Answer 1

<?php
    date_default_timezone_set('Asia/Karachi');
    $date=getdate(date("U"));
    $day = $date['mday'];
    $month =$date['mon'];
    $year = $date['year'];
    $currentDate = $year.'-'.$month.'-'.$day;
?>

Answer 2

Use for example moment.js to dertmine the current timezone offset for CET but this way you get the summer&winter offsets

var offsetCETmillisec = moment.tz.zone('Europe/Berlin').offset(moment())* 60 * 1000;

  $group: {
    _id: {
      'year': {'$year': [{ $subtract: [ '$createdAt', offsetCETmillisec ]}] },
      'month': {'$month': [{ $subtract: [ '$createdAt', offsetCETmillisec ]}] },
      'day': {'$dayOfMonth': [{ $subtract: [ '$createdAt', offsetCETmillisec ]}] }
    },
    count: {$sum: 1}
  }
}