- Updates all datetime references to use WordPress format

- Removes depreciated datetime conversion method from Helpers
- Fixes translation & error display issue in import
Closes #432

lib/Util/Helpers.php

@@ -2,77 +2,6 @@
 namespace MailPoet\Util;
 
 class Helpers {
-  
-  /*
-   * Matches each symbol of PHP date format standard
-   * with jQuery equivalent codeword
-   * @author Tristan Jahier
-   */
-  static function dateformat_PHP_to_jQueryUI($php_format) {
-    $SYMBOLS_MATCHING = array(
-      // Day
-      'd' => 'dd',
-      'D' => 'D',
-      'j' => 'd',
-      'l' => 'DD',
-      'N' => '',
-      'S' => '',
-      'w' => '',
-      'z' => 'o',
-      // Week
-      'W' => '',
-      // Month
-      'F' => 'MM',
-      'm' => 'mm',
-      'M' => 'M',
-      'n' => 'm',
-      't' => '',
-      // Year
-      'L' => '',
-      'o' => '',
-      'Y' => 'yy',
-      'y' => 'y',
-      // Time
-      'a' => '',
-      'A' => '',
-      'B' => '',
-      'g' => '',
-      'G' => '',
-      'h' => '',
-      'H' => '',
-      'i' => '',
-      's' => '',
-      'u' => ''
-    );
-    $jqueryui_format = "";
-    $escaping = false;
-    for ($i = 0; $i < strlen($php_format); $i++) {
-      $char = $php_format[$i];
-      if($char === '\\') // PHP date format escaping character
-      {
-        $i++;
-        if($escaping) {
-          $jqueryui_format .= $php_format[$i];
-        } else {
-          $jqueryui_format .= '\'' . $php_format[$i];
-        }
-        $escaping = true;
-      } else {
-        if($escaping) {
-          $jqueryui_format .= "'";
-          $escaping = false;
-        }
-        if(isset($SYMBOLS_MATCHING[$char])) {
-          $jqueryui_format .= $SYMBOLS_MATCHING[$char];
-        } else {
-          $jqueryui_format .= $char;
-        }
-      }
-    }
-    
-    return $jqueryui_format;
-  }
-
   static function getMaxPostSize($bytes = false) {
     $maxPostSize = ini_get('post_max_size');
     if(!$bytes) return $maxPostSize;