Date Field shows different times in Frontend and Backend

Question

I have a date field applied to a node. When I set the time in the date field to i.e. 10am and render that date to my website like so:

echo  date('d.m.Y H:i', strtotime($variables['field_date'][0]['value']));

The output is 9:00am. There is only one single timezone set for the whole website, no individual timezones for users are activated.