If I understand what you're asking:

You want to be able to manipulate a Date global Variable to where you can enter in the date (if the date isn't coming from the database).  Just set up a global variable named "gv_Date" and you can do an ActiveX Script for that with the following:

Function Main()

 dim a

 a=inputbox("What is the date?")

 if a <> "" then

  DTSGlobalVariables("gv_Date").Value=a

 end if

 Main = DTSTaskExecResult_Success

End Function