Added extension for ISO 8601 date-time format

src/Cloudflare/Extensions/DateTimeExtensions.cs

@@ -0,0 +1,34 @@
+namespace AMWD.Net.Api.Cloudflare
+{
+	/// <summary>
+	/// Extension methods for <see cref="DateTime"/>s.
+	/// </summary>
+	public static class DateTimeExtensions
+	{
+		private const string Iso8601Format = "yyyy-MM-dd'T'HH:mm:ss'Z'";
+
+		/// <summary>
+		/// Converts the specified <see cref="DateTime"/> to its ISO 8601 string representation.
+		/// </summary>
+		/// <remarks>
+		/// This method ensures that the resulting string conforms to the ISO 8601 standard, which is
+		/// commonly used for representing dates and times in a machine-readable format.
+		/// </remarks>
+		/// <param name="dateTime"> The <see cref="DateTime"/> instance to convert.</param>
+		/// <returns>
+		/// A string representing the <paramref name="dateTime"/> in ISO 8601 format.
+		/// <br/>
+		/// If the <see cref="DateTime.Kind"/> property is <see cref="DateTimeKind.Local"/>, the value is first converted to <see cref="DateTimeKind.Utc"/>.
+		/// <br/>
+		/// If the <see cref="DateTime.Kind"/> is <see cref="DateTimeKind.Unspecified"/>, it is treated as <see cref="DateTimeKind.Utc"/>.
+		/// </returns>
+		public static string ToIso8601Format(this DateTime dateTime)
+		{
+			if (dateTime.Kind == DateTimeKind.Local)
+				return dateTime.ToUniversalTime().ToString(Iso8601Format);
+
+			// DateTimeKind.Unspecified is treated as UTC
+			return dateTime.ToString(Iso8601Format);
+		}
+	}
+}

src/Extensions/Cloudflare.Zones/Internals/InternalRemoveZoneHoldFilter.cs

@@ -9,7 +9,7 @@
 			var dict = new Dictionary<string, string>();
 
 			if (HoldAfter.HasValue)
-				dict.Add("hold_after", HoldAfter.Value.ToUniversalTime().ToString("yyyy-MM-dd'T'HH:mm:ss'Z'"));
+				dict.Add("hold_after", HoldAfter.Value.ToIso8601Format());
 
 			return dict;
 		}

test/Cloudflare.Tests/Extensions/DateTimeExtensionsTest.cs

@@ -0,0 +1,50 @@
+using AMWD.Net.Api.Cloudflare;
+
+namespace Cloudflare.Tests.Extensions
+{
+	[TestClass]
+	public class DateTimeExtensionsTest
+	{
+		private const string Format = "yyyy-MM-dd'T'HH:mm:ss'Z'";
+
+		[TestMethod]
+		public void ShouldReturnIsoStringForUtc()
+		{
+			// Arrange
+			var dateTime = new DateTime(2025, 10, 10, 1, 2, 3, DateTimeKind.Utc);
+
+			// Act
+			string isoString = dateTime.ToIso8601Format();
+
+			// Assert
+			Assert.AreEqual(dateTime.ToString(Format), isoString);
+		}
+
+		[TestMethod]
+		public void ShouldReturnIsoStringForUnspecified()
+		{
+			// Arrange
+			var dateTime = new DateTime(2025, 10, 10, 1, 2, 3, DateTimeKind.Unspecified);
+
+			// Act
+			string isoString = dateTime.ToIso8601Format();
+
+			// Assert
+			Assert.AreEqual(dateTime.ToString(Format), isoString);
+		}
+
+		[TestMethod]
+		public void ShouldReturnIsoStringForLocal()
+		{
+			// Arrange
+			var dateTime = new DateTime(2025, 10, 10, 1, 2, 3, DateTimeKind.Local);
+			var offset = TimeZoneInfo.Local.GetUtcOffset(dateTime);
+
+			// Act
+			string isoString = dateTime.ToIso8601Format();
+
+			// Assert
+			Assert.AreEqual(dateTime.Subtract(offset).ToString(Format), isoString);
+		}
+	}
+}