How to reset a date column value to null in a list workflow action???

Hello,

I am using Sharepoint 2010 and ran into an issue where I have a date column in my list.  If another field is filled out, I add the current date to the date field in my list workflow action.  If the other field value is later removed, I wanted to remove the date value as well but I can't seem to find a way to set the date column to be empty/null again within the list workflow action.  Any clues?

Thanks - Peter