String was not recognized as a valid DateTime.why?

Question

while i am trying to fetch datetime data from db it shows the error  String was not recognized as a valid DateTime.

 

 

code:

 

Model: 

 public DateTime? ReviewedDate{get;set;}

 

Controller: 

DateTime? ReviewedDate = null;

if (testordervm.ReviewedDate != DateTime.MinValue)

{

ReviewedDate = DateTime.Parse(dsobj.Tables[0].Rows[0]["ReviewedDate"].ToString());

}

else

{

ReviewedDate = null;

}