Unit test error : This function can only be invoked from LINQ to Entities

I am coding a MVC 5 internet application, and I have an expression as follows:

public Expression<Func<Account, bool>> IsExpiresDateTimeLessThanMinimumDaysLeftInFreeTrialSubscription(int minimumDaysLeftInSubscriptionForEmail)
{
    return Account => System.Data.Entity.DbFunctions.DiffHours(Account.freeTrialEndDate, DateTime.UtcNow) < minimumDaysLeftInSubscriptionForEmail;
}

When retrieving data from the database, the above expression completes correctly. However, when writing a unit test that uses the above expression I am getting the following error:

This function can only be invoked from LINQ to Entities

I gather that this is because the System.Data.Entity.DbFunctions.DiffHours function converts the expression into code that only a database system can understand.

Because of the above fact, is it possible to unit test the above expression when using a mock repository that uses a List rather than a DbSet? If not, how should I unit test any code that uses the expression? Is it possible to unit test the expression?

Thanks in advance.