I am trying to fetch all records added today using DateDiff SQL syntax via Linq expression in MVC 5 / Entity Framework 6 application. DateDiff function throw runtime error>
Your code is never executed in the database, the "issue" is that Linq.Dynamic tries to parse it as C# code, where it fails. As far as I know it is not possible to call SQL with dynamic linq.
I believe you are looking for is using raw SQL, not .NET code which Linq.Dynamic is, this page at MSDN will give you more information about using raw SQL.