How to take the Database Backup for a Specified Date

Hello every one, i have developed a Software in C#.Net which will take the Backup of a Database in two ways Fully and Differential type. Now i want to make the Backing Up of Database for a Specified Date. If user selects one date other than the Current Date the app should take the Backup of the Database Specified on the Date Selected before the Current Date. I am trying for this, pls can any one suggest me how to work it out.
  
                    Thankx in Advance.