How to create a dynamic calendar in C#? Just like system calendar

I am creating trail version of my Application I give 2 months trial period for my App but when the user change the system time trial period is updated to prevent this problem I want to create my own calendar and use it is date if you have any solution to my problem please............ help me.
Note: if you have any other solution to solve my problem please help me

thanks advance!

DateTime todayDate = DateTime.Now;
DateTime after3MonthDate = todayDate.AddMonths(3);
//Set First Day of Month
after3MonthDate = new DateTime(after3MonthDate.Year, after3MonthDate.Month, 1);