app.cs

#define NONEST

void Main()
{
    Database.SetInitializer<MyContext>(null);
    using (var db = new MyContext())
    {
        var date = "20180315";
        var query = from row in db.Tabl1
                    where SqlFunctions.DateDiff(
                        "dd",
                        row.TheDate,
                        date.Substring(0, 4) + "-" + date.Substring(5, 2) + "-" + date.Substring(7, 2)
                    ) > 0
                    select row;

        query.ToString().Dump();
    }
}

public class Tabl1
{
    public int Id { get; set; }
    public DateTime TheDate { get; set; }
}

public class MyContext : DbContext
{
    public DbSet<Tabl1> Tabl1 { get; set; }
}

generated sql.sql

SELECT 
    [Extent1].[Id] AS [Id], 
    [Extent1].[TheDate] AS [TheDate]
    FROM [dbo].[Tabl1] AS [Extent1]
    WHERE (DATEDIFF(dd, [Extent1].[TheDate], CASE WHEN (SUBSTRING(@p__linq__0, 0 + 1, 4) IS NULL) THEN N'' ELSE SUBSTRING(@p__linq__0, 0 + 1, 4) END + N'-' + CASE WHEN (SUBSTRING(@p__linq__1, 5 + 1, 2) IS NULL) THEN N'' ELSE SUBSTRING(@p__linq__1, 5 + 1, 2) END + N'-' + CASE WHEN (SUBSTRING(@p__linq__2, 7 + 1, 2) IS NULL) THEN N'' ELSE SUBSTRING(@p__linq__2, 7 + 1, 2) END)) > 0