The datepicker will create a timezone local Date object, transforming it to JSON for sending it will convert it to UTC, thus rolling back your 2 hours of GMT+2 over the dateline.
Working with Date can be a bit tricky, I suggest looking into libraries like Moment.js to be on the safe side when transforming dates and times. By this you can freely define the format of your desired output string.
See also: How to send AngularStrap datepicker value without timezone?