ExpirationSkipConvention.cs

[AttributeUsage(AttributeTargets.Method, Inherited = false, AllowMultiple = false)]
public class SkipAttribute : Attribute
{
    public string ExpirationDate { get; set; }

    public bool HasExpired
    {
        get
        {
            DateTime expirationDate;
            return DateTime.TryParse(ExpirationDate, out expirationDate)
                && DateTime.Now > expirationDate;
        }
    }
}

public class CustomConvention : Convention
{
    public CustomConvention()
    {
        Classes
            .NameEndsWith("Tests");

        Methods
            .Where(method => method.IsVoid());

        CaseExecution
            .Skip(@case => {
                if (@case.Method.HasOrInherits<SkipAttribute>()) {
                    var skipAttribute = @case.Method.GetCustomAttributes<SkipAttribute>(true).Single();

                    return !skipAttribute.HasExpired;
                }

                return false;
            });
    }
}