i am using v3.1.3 and i had to use data('DateTimePicker')
like this
var fromE = $( "#" + fromInput );
var toE = $( "#" + toInput );
$('.form-datepicker').datetimepicker(dtOpts);
$('.form-datepicker').on('change', function(e){
var isTo = $(this).attr('name') === 'to';
$( "#" + ( isTo ? fromInput : toInput ) )
.data('DateTimePicker')[ isTo ? 'setMaxDate' : 'setMinDate' ](moment($(this).val(), 'DD/MM/YYYY'))
});