change from Textbox to datetimepicker

[C# Programming]

Hi, I am making a software,in that want to change month of a datetimepicker using a textbox.
i.e. as i put 15 in textbox the value of the month of a datetimepicker is increased by 15months.
like current date is 05/10/2013
and when i put 15 in a text box...the value of datetimepicker is increased by 05/01/2015.

thanks..
waiting for your reply..