I'm using this for my date in LS and is working properly.
date {
locale => "en"
match => ["returnDate", "dd.MM.YYYY"]
timezone => "Europe/Ljubljana"
target => "returnDate"
}
Maybe it isn't working because you left out locale property.
After your example you'll have a field timestamp and a field Time. I'm assuming you don't need Time any more so just unset/remove it.