public async Task <IActionResult> OnPost([FromQuery] int year, [FromQuery] int month, [FromQuery] int day)
        {
            if (!ModelState.IsValid)
            {
                ErrorMessage = "入力に誤りがあります。";
                return(PageResult(year, month, day));
            }
            var startTime = LocalTimePattern.Create("HH:mm", CultureInfo.CurrentCulture).Parse(StartTime !);

            if (!startTime.Success)
            {
                ErrorMessage = "入力に誤りがあります。";
                return(PageResult(year, month, day));
            }
            var date   = new LocalDate(year, month, day);
            var result = await _service.CreateAppointmentSlotAsync(date.At(startTime.Value), Period.FromMinutes(DurationMinutes !.Value), CountOfSlot !.Value);

            if (!result.Succeeded)
            {
                ErrorMessage = result.ErrorMessage;
                return(PageResult(year, month, day));
            }
            return(RedirectToPage("Index", new { year, month, day }));
        }