You could also set a flag Default them to 0 first of course.

my $Have_Date=0;
my $Have_Time=0;
[download]

Then set $Have_Date=1; or $Have_Time=1; when the user enters the info. Then when its time to do the query:

if ($Have_Date)
    {
    #
    # Bind Date Param Stuff
    #
    }
if ($Have_Time)
    {
    #
    # Bind Time Param Stuff
    #
    }
[download]

You get the idea...