If you want to get the date and time from $_POST["date"]
and $_POST["time"]
Use following
DateTime::createFromFormat("d.m.Y H:i", $_POST["date"]." ".$_POST["time"]);
If you want to kow the number of seconds from midnight for that particular time you can however use this,
DateTime::createFromFormat("Ymd H:i", date("Ymd")." ".$_POST["time"]);