date_timezone_set() expects parameter 1 to be DateTime, boolean given

According to mikeryan's will (in #1021076: Support for date module in Drupal 7), this is a new issue about date migration.

Did you encounter the following error for any of your tests ? If so, how did you get rid of them?
date_timezone_set() expects parameter 1 to be DateTime, boolean given File common.inc, line 1905
The only time your code seem to call date_timezone_set is in date.test file, but it is called indeed in common.inc inside format_date function with :

<?php
function format_date($timestamp, $type = 'medium', $format = '', $timezone = NULL, $langcode = NULL) {
... (no modification of $timestamp value)
  // Create a DateTime object from the timestamp.
  $date_time = date_create('@' . $timestamp);
  // Set the time zone for the DateTime object.
  date_timezone_set($date_time, $timezones[$timezone]);
...
}
?>

Moreover, don't you think the following piece of code (around line #100) of date.inc is problematic in that way ?

<?php
public function prepare($entity, array $field_info, array $instance, array $values) {
..
 // Work from a timestamp
      $from = MigrationBase::timestamp($from);
      if ($to) {
        $to = MigrationBase::timestamp($to);
      }

      // What does the destination field expect?
      switch ($field_info['type']) {
        case 'datestamp':
          // Already done
          break;
        case 'datetime':
          // YYYY-MM-DD HH:MM:SS
          $from = format_date($from, 'custom', 'Y-m-d H:i', $timezone);
          if ($to) {
            $to = format_date($to, 'custom', 'Y-m-d H:i', $timezone);
          }
          break;
        case 'date':
          // ISO date: YYYY-MM-DDTHH:MM:SS
          $from = format_date($from, 'custom', 'Y-m-d\TH:i', $timezone);
          if ($to) {
            $to = format_date($to, 'custom', 'Y-m-d\TH:i', $timezone);
          }
          break;
        default:
          break;
      }
?>

Indeed, if($to == TRUE), $to = MigrationBase::timestamp($to) = (is_numeric($to) ? $to) = $to = TRUE, and so on each time
I tried with if(isset($to)) but it didn't change anything for me.