getting date from string in object

getting date from string in object

I save my dates as datetime in my db (e.g. 2013-09-23 15:00:00).
When I create an object I call this date by
$event->start
When I just echo it, I get the right value but it is a string and no
longer a datetime. That is why this won't work:
date_format('d-m-Y', $event->start)
To use date() I need a timestamp, which I don't have. What is a possible
solution?