[DatePicker] Prevent silently ignoring date formatting failures

src/Core/Components/DateTime/FluentCalendar.razor.cs

@@ -293,9 +293,9 @@ private async Task PickerYearSelectAsync(DateTime? year)
     /// <summary />
     protected override bool TryParseValueFromString(string? value, out DateTime? result, [NotNullWhen(false)] out string? validationErrorMessage)
     {
-        BindConverter.TryConvertTo(value, Culture, out result);
-        validationErrorMessage = null;
-        return true;
+        bool success = BindConverter.TryConvertTo(value, Culture, out result);
+        validationErrorMessage = success? null : $"The {DisplayName ?? (FieldBound ? FieldIdentifier.FieldName : UnknownBoundField)} field is not in a valid format.";
+        return success;
     }
 
     /// <summary />

src/Core/Components/DateTime/FluentDatePicker.razor.cs

@@ -145,10 +145,9 @@ protected override bool TryParseValueFromString(string? value, out DateTime? res
             value = new DateTime(year, 1, 1).ToString(Culture.DateTimeFormat.ShortDatePattern);
         }
 
-        BindConverter.TryConvertTo(value, Culture, out result);
-
-        validationErrorMessage = null;
-        return true;
+        bool success = BindConverter.TryConvertTo(value, Culture, out result);
+        validationErrorMessage = success ? null : $"The {DisplayName ?? (FieldBound ? FieldIdentifier.FieldName : UnknownBoundField)} field is not in a valid format.";
+        return success;
     }
 
     private string PlaceholderAccordingToView()