Momentjs.com has good documentation on how to manipulate the date/time in relation to the current moment. Since Momentjs is required for the Datetimepicker, might as well use it.
var startDefault = moment().startof('day').add(1, 'minutes');
$('#startdatetime-from').datetimepicker({
defaultDate: startDefault,
language: 'en',
format: 'yyyy-MM-dd hh:mm'
});