Added workaround for strtotime("0000-00-00 00:00:00") returning -6216995...

Conform docs (http://api.cakephp.org/2.3/class-CakeTime.html#_format) CakeTime::format should return the passed default when invalid date is passed (for example: 0000-00-00).
This patch will fix the function when used on 64 bit system, because strtotime will return a negative integer on those systems for date strings with only zeros.