why viewstate variable data updated on button click but not on page refresh

i have one web page in this one button control i have added.i am for first time postback i am assing current date to one session variable.In pre render method i am asssigning it to viewstate variable.on button click i am matching this to variable value if it match i am changing session varible value to current time so now there is different time in session and viewstate variable.After buttonclick method executed prerender method is called in this viewstate varible value set to session variable value.if now i am click on submit button again i found that my viewstate and session varible values are same but in case of page refresh it not same.my code is

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not IsPostBack Then
Session("Time") = Date.Now.ToString
End If
End Sub

Private Sub Statemanagement_PreRender(sender As Object, e As System.EventArgs) Handles Me.PreRender
ViewState("Time") = Session("Time")
End Sub


Protected Sub btnLogin_Click(sender As Object, e As EventArgs) Handles btnLogin.Click


If ViewState("Time") = Session("Time") Then
MsgBox("Match")
Session("Time") = Date.Now.ToString
End If
End sub