There's no framework method that will handle this. You can use P/Invoke to call a Win32 API to set the system date/time.
In your form level code, add
Private Structure SYSTEMTIME
Public wYear As Short
Public wMonth As Short
Public wDayOfWeek As Short
Public wDay As Short
Public wHour As Short
Public wMinute As Short
Public wSecond As Short
Public wMilliseconds As Short
End Structure
Private Declare Function SetSystemTime Lib "kernel32" (ByRef lpSystemTime As SYSTEMTIME) As Boolean
Then in a method, use code such as this to set the date.
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim lpSystemTime As SYSTEMTIME
lpSystemTime.wDay = 12
lpSystemTime.wMonth = 7
lpSystemTime.wYear = 2002
' Fill the structure and call
SetSystemTime(lpSystemTime)
End Sub