Possible Duplicate:
Refresh Excel VBA Function Results
I have simple function in excel:
Function Test() As String
Test = Now
End Function
and I am able to use it as:

Now why is it that when I press the button:

the value does not update !?
For example if I have a cell with the formula =Rand() every time I click the calculate value I will get a different number. How can I make my custom function behave the same way ?