How to add or subtract days from a calender ...???

I have taken a datepicker and a textbox, select a date(DD/MM/YY) from datepicker, If i enter in a textbox + or - days like (+7D or -5D) the addition days or substraction days from selected date in date picker , will be store in database using  mvc architechture .kindly help me please ,i have no idea about it.