why wont it format into a ToShortDateString?

Hi
 
I have the following line of code
 
return Convert.ToDateTime(date.Value).ToShortDateString();
 
I get the error Cannot implicitly convert type 'string' to 'System.DateTime?'        
Can anyone please let me know where I am going wrong and how I can resolve this problem?
I need it to format it in the shortdateString format because when I remove that bit it includes the time which I do not want.
 
Thank you very much
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