Removing time from DateTime in RSS Feed

Hi.
I have cod which pulls data from an MS SQL database and displays it in an RSS feed.
To display the date each article was uploaded to the db, I am using the following line:
(the data type of the field in the db is "Date", not "DateTime")
xtwFeed.WriteElementString("pubDate", ((DateTime)rdr1["last_modified"]).ToString("yyyyMMdd"));  
Replacing DateTime with Date produces an error: "The type or NameSpace "Date" could not be found..." 
My problem is that the output from this includes the time. I don't want the time, just the date.
How can I rewrite this line to remove the time element.
 
Thanks
 
 
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