I am trying a simple ClientAPI callback app, but have come unstuck with sharing a variable between Clientside and serverside. 

I am trying to pass a string variable from the onclick method of a button to the server RaiseClientAPICallBackEvent.  I code

dim szPrev as string
dim startdate as datetime

then in the Page Load I say

   ClientAPI.RegisterClientVariable( Page, szPrev, startdate.AddMonths(-1).ToString(), True )

but I find this has NOT set the value of szPrev as I expected, so I add

   szPrev = startdate.AddMonths(-1).ToString()

Still in the Page_Load I add

 but on the serverside event, I find that eventArgument is undefined.

Am I missing something simple?  TIA!