dynamic layer filter

[Mauricio Stange H.]

Hi,
    I'm trying to make a date filter for the information that display a
layer of the mapserver, so that the user select a month, or a year and
then press a refresh button. I know how to set the filter manually.
    But how can I make this dynamically. It is possible to do this with
the hidden var QueryString??

Thanks,
    Mauricio Stange H.