public static IRuleBuilderOptions <T, string> DateValidation <T>(this IRuleBuilder <T, string> rule)
 {
     return(rule
            .NotEmpty()
            .WithMessage("Order date cannot be empty.")
            .MustAsync(async(orderDate, cancellation) => await ValidationHelpers.BeValidDateFormat(orderDate))
            .WithMessage("Order date must be of format: d.MM.yyyy e.g. 12.02.2012"));
 }