How to validate masktextbox

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C#

C#3.5

hi all,

actually i have one application where having three column 1. start_amout taking date 2)validity 3) amount_return_date

in validity field iam using combobox having 1 to 12 months, now i want if user select start_amount_taking_date suppose today that is 03/03/2015 and selected validity 2 months so automatically, amount_return_date should display 2 months date that is 03/05/2015 if user selected 12 months so amount_return_date should be display 03/03/2017 actually frankly speaking iam not getting how should i do because both (1) and (3) are masktextbox and validity is combobox

so please anyone can help me ,please

What I have tried:

if (date.Day != DateTime.DaysInMonth(date.Year, date.Month))
      return date.AddMonths(1);
   else 
      return date.AddDays(1).AddMonths(1).AddDays(-1);

i know this code but how i implemented into my situation is not getting