override default DateTime format in C#

I am working in an application built in .NET framework 2.0 but I am running it in .NET framework 4.0, with SQL server as a database.

when I try to take date as a input from Calendar controller (Webforms) its in the format dd/MM/yyyy and this application using Convert.ToDateTime() to parse this date and it throws exception.

so how can I resolve this problem and is there any way that DateTime class object store the date in specific format means I want to store date in dd/MM/yyyy format even when I copy this date object to another, then both object should have same format.

and sorry for My English :p