Added strong DateOnly type

Author: Peter

src/KingpinNet/CommandLineItem.cs

@@ -147,8 +147,10 @@ internal T ConvertToType(string value)
         }
         else if (ValueType == ValueType.Date)
         {
-            if (DateTime.TryParseExact(value, new[] { "yyyy.MM.dd", "yyyy-MM-dd", "yyyy/MM/dd" }, CultureInfo.InvariantCulture, DateTimeStyles.None, out var result))
-                return (T)Convert.ChangeType(result, typeof(DateTime));
+            if (typeof(T) == typeof(DateOnly) && DateOnly.TryParseExact(value, new[] { "yyyy.MM.dd", "yyyy-MM-dd", "yyyy/MM/dd" }, CultureInfo.InvariantCulture, DateTimeStyles.None, out var resultDate))
+                return (T)Convert.ChangeType(resultDate, typeof(T));
+            else if (DateTime.TryParseExact(value, new[] { "yyyy.MM.dd", "yyyy-MM-dd", "yyyy/MM/dd", "yyyy/MM/ddTHH:mm:ss", "yyyy-MM-ddTHH:mm:ss",  "yyyy.MM.ddTHH:mm:ss" }, CultureInfo.InvariantCulture, DateTimeStyles.None, out var result))
+                return (T)Convert.ChangeType(result, typeof(T));
             else
                 throw new ArgumentException($"'{value}' is not a date", nameof(value));
         }

src/KingpinNet/ValueType.cs

@@ -26,6 +26,8 @@ internal static ValueType Convert(Type type)
             return ValueType.Int;
         if (type == typeof(Int64))
             return ValueType.Long;
+        if (type == typeof(DateOnly))
+            return ValueType.Date;
         if (type == typeof(DateTime))
             return ValueType.Date;
         if (type == typeof(TimeSpan))

test/UnitTests/ParserTests.cs

@@ -253,7 +253,21 @@ public void ParseDurationSuccess()
             // Assert
             Assert.Equal("1:00:00", result.Result["time"]);
         }
+        [Fact]
+        public void ParseDateSuccess()
+        {
+            // Arrange
+            string[] args = new[] { "--date=2026-12-27" };
+            var application = new KingpinApplication(consoleMock.Object);
+            var flag = application.Flag<DateOnly>("date", "");
+
+            // Act
+            var subject = new Parser(application);
+            var result = subject.Parse(args);
 
+            // Assert
+            flag.Value.Equals(DateOnly.Parse("2026-12-27"));
+        }
         [Fact]
         public void ParseDurationSuccessWithDays()
         {