How to clear a date field ?

How can I clear a date field ? Null does not work, nor does empty string "", nor can I clear picklist as it is a date field.
e.g.:
if((input.Status == "Closed"))
{
set ClosedDate = zoho.currenttime;
}
else
{
set ClosedDate = null;
}

Thanks, and I love the Zoho 2 features & look !
markv