LINQ to Entities does not recognize the method 'System.String ToString(System.Object)' method error occurs with nullable fields

I have an entity framework's EDMX generated class having a DOB (Date Of Birth) property.

public partial class Contact : EntityBase
{
    public Contact()
    {
    }

    public Nullable<System.DateTime> DOB { get; set; }
}

I created a LINQ expression for searching records using a string value "2015-02-21" against DOB.

 Expression<Func<Contact, bool>> cntExpression = p => Convert.ToString(p.DOB).ToLower().Trim().Contains("2015-02-21");

I used business logic class's method to filter records using the above LINQ expression.

IQueryable<Contact> qryContact = _cntMgr.GetFiltered(cntExpression);

But as you can see that DOB is a nullable property, so it throws error when below code starts looping through records existing in the above IQueryable instance.

foreach (var contact in qryContact)
            {
                if (contact!=null)
                {
                    // some code gets execute here..
                }
            }

The error I get is this:

 LINQ to Entities does not recognize the method 'System.String ToString(System.Object)' method, and this method cannot be translated into a store expression.

I already know that System.String ToString isn't supported in Linq to Entities, but I need a solution or a workaround for this problem.

Kindly help me in fixing this issue.

Thanks in advance.