jqueyr datetimepicker set to date depand on from date

I am using jQuery "datetimepicker" and I want to set the "to date" value depend on the "from date" value and the Select box value which contains following values.

1- Weekly (7+ days)
2- Monthly (30+ days)
3- Half Yearly (6+ months)
3- Yearly (1+ Year)

Example:

1- Select From Date: 2015-05-29
2- Duration Monthly
3- To date should be 2015-06-29

I am using following code to select date start date.

jQuery('#start_date').datetimepicker({
    format:'m/d/Y',
    closeOnDateSelect:true,
    timepicker:false
});

Please suggest.

Thanks