System.Data.Objects.EntityFunctions和System.Data.Objects.SqlClient.SqlFunctions中的方法进行比较，如下
where System.Data.Objects.SqlClient.SqlFunctions.DateDiff("s",DateTime.Now,u.Time)>0
where System.Data.Objects.EntityFunctions.DiffSeconds(DateTime.Now,u.Time)>0
LINQ to Entities does not recognize the method 'System.DateTime ToDateTime(System.String)' method
原文：http://www.cnblogs.com/zcm123/p/6247281.html