Add first_operation_date method and update date handling in Finances class

autoload/factory/class.Finances.php

@@ -2,6 +2,12 @@
 namespace factory;
 class Finances
 {
+  static public function first_operation_date()
+  {
+    global $mdb;
+    return $mdb -> get( 'finance_operations', 'date', [ 'ORDER' => [ 'date' => 'ASC' ] ] );
+  }
+
   static public function get_operation_tags( int $operation_id )
   {
     global $mdb;