updating null or empty value in smartbox date tyep colum from smartobject event workflow

I want to set the emty value in smartbox datetime type column form the smartobject event used in worklfow.

1. created datetime type datafield, assinged it empty date as default value.

2. used the smartobject event and save mathod to update the datetime column in smartbox

3. used the above datefield colum in save method.

But the date is not changing to empty.