Web Forms :: Setting Value Of Asp:textbox In Javascript In 3.5

I created a page with several controls (ddl, tbx) that I use for filtering a Gridview. I created a javascript function that runs on the OnClientClick Event of an <asp:button to set the value of an <asp:TextBox to today and a Selected Value in a ddl.

function setMyToday() {
var tbxToday = document.getElementById('ctl00_cntPage_tbxToday');
var tbxUID = document.getElementById('ctl00_cntPage_tbxUserID');
var ddlPV = document.getElementById('ctl00_cntPage_ddlProvider');
tbxToday.value = makeToday();
ddlPV.value = tbxUID.value;
}

The button submits the page and returns some records in a Gridview. I can see the value in the tbxToday when the button is clicked and the row in the drop down list, but when the page is posted,the value in the text bos is lost and as a result is not used to filter the records. However the value in the ddlProvider is maintained.