how to convert date into timestamp using timezone

I am using strtotime to convert date into time stamp but its does not work correctly. I know its relatively to time zone. My drupal store time zone is denver. I am not sure why its not showing correct date. there is around one day difference. so The last date I used 31 but I get last date of 30th not 31

Here are my query to make date into time and fetch record

date_default_timezone_set(drupal_get_user_timezone());
$startdate = strtotime(\Drupal::request()->query->get('startdate')); // 2021-12-01
$endate = strtotime(\Drupal::request()->query->get('enddate')); //2021-12-31
$database = \Drupal::database();
if(!empty($startdate)){
$userrecord = $database->query("SELECT mail, access FROM users_field_data WHERE access BETWEEN $startdate and $endate")->fetchAll(); 

Here is twig where i Called date

{{ result.access|format_date('custom', 'F j, Y')|e }}

Drupal stores times always in UTC, never in local time. Also Unix timestamps are by definition in UTC.

You can convert a local date string to a timestamp by creating a \DateTime object:

$timestamp = (new \DateTime('2021-12-31'))->getTimestamp();

Which is using the default PHP timezone or by specifying your own time zone:

$timestamp = (new \DateTime('2021-12-31', new \DateTimeZone('America/New_York')))->getTimestamp();