If website is on timezone other than UTC, editing Event Series causes the "Confirm Date Changes" prompt to incorrectly appear

This fixes it for me. It looks like when building the $diff_array for comparing the original entity datetime and the values from the form, the timezone changes were causing a potentially wrong date.

For the recurring events, the time was being added to the UTC version of the date, which could be a different date from the website timezone date.

Example from DailyRecurringDate.php. The $start_timestamp was being created from the correct time -- because the time is its own separate value. But the $user_input['daily_recurring_date'][0]['value'] is in UTC time and could produce the wrong date -- e.g. Vancouver has an 8-hour time difference so the UTC date could be a day ahead. That means the $start_timestamp could be wrong.

    $user_timezone = new \DateTimeZone(date_default_timezone_get());
    $user_input = $form_state->getValues();

    $time = $user_input['daily_recurring_date'][0]['time'];
    if (is_array($time)) {
      $temp = DrupalDateTime::createFromFormat('H:i:s', $time['time']);
      $time = $temp->format('h:i A');
    }
    $time_parts = static::convertTimeTo24hourFormat($time);
    $timestamp = implode(':', $time_parts);

    $start_timestamp = $user_input['daily_recurring_date'][0]['value']->format('Y-m-d') . 'T' . $timestamp;

I've done the following changes to make sure the date is in the right timezone before the time is attached to it:

    $user_timezone = new \DateTimeZone(date_default_timezone_get());
    $user_input = $form_state->getValues();

    $time = $user_input['daily_recurring_date'][0]['time'];
    if (is_array($time)) {
      $temp = DrupalDateTime::createFromFormat('H:i:s', $time['time']);
      $time = $temp->format('h:i A');
    }
    $time_parts = static::convertTimeTo24hourFormat($time);
    $timestamp = implode(':', $time_parts);

    $user_input['daily_recurring_date'][0]['value']->setTimezone($user_timezone);
    $start_timestamp = $user_input['daily_recurring_date'][0]['value']->format('Y-m-d') . 'T' . $timestamp;

See the patch for all changes. Custom events was a bit different as there ended up being too many timezone changes in the code that was also causing discrepancies.

I have tested on different timezone settings for the website's default timezone.

@owenbush have you tried on simplytest.me with exactly my settings? I can replicate it every time with those settings

timezones are THE biggest pain in the Drupal butt