fix: encode RFC 3339 UTC offset as "Z" instead of "+0000"

Sources/KarrotCodableKit/BetterCodable/DateValue/Strategy/RFC3339Strategy.swift

@@ -20,7 +20,7 @@ public struct RFC3339Strategy: DateValueCodableStrategy {
     let dateFormatter = DateFormatter()
     dateFormatter.timeZone = TimeZone(secondsFromGMT: 0)
     dateFormatter.locale = Locale(identifier: "en_US_POSIX")
-    dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZ"
+    dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ"
     return dateFormatter
   }()
 

Tests/KarrotCodableKitTests/BetterCodable/DateValue/RFC3339StrategyTests.swift

@@ -0,0 +1,99 @@
+//
+//  RFC3339StrategyTests.swift
+//  KarrotCodableKit
+//
+//  Created by Elon on 7/13/26.
+//
+
+import Foundation
+import Testing
+
+import KarrotCodableKit
+
+@Suite("RFC3339Strategy")
+struct RFC3339StrategyTests {
+
+  private struct Fixture: Codable {
+    @DateValue<RFC3339Strategy> var date: Date
+  }
+}
+
+// MARK: - Encoding a Date
+
+extension RFC3339StrategyTests {
+
+  @Test
+  func encodesUTCDateWithZOffsetInsteadOfRFC822Offset() {
+    // given
+    let date = Date(timeIntervalSince1970: 1715082540) // 2024-05-07T11:49:00 UTC
+
+    // when
+    let result = RFC3339Strategy.encode(date)
+
+    // then
+    // RFC 3339 allows only "Z" or "+00:00" for UTC, never the colon-less "+0000".
+    #expect(result == "2024-05-07T11:49:00Z")
+  }
+
+  @Test
+  func serializesUTCDateWithZOffsetViaJSONEncoder() throws {
+    // given
+    let date = Date(timeIntervalSince1970: 1715082540) // 2024-05-07T11:49:00 UTC
+    let fixture = Fixture(date: date)
+
+    // when
+    let data = try JSONEncoder().encode(fixture)
+    let json = try #require(String(bytes: data, encoding: .utf8))
+
+    // then
+    #expect(json.contains("\"2024-05-07T11:49:00Z\""))
+    #expect(!json.contains("+0000"))
+  }
+}
+
+// MARK: - Decoding an RFC 3339 string
+
+extension RFC3339StrategyTests {
+
+  @Test(arguments: [
+    "2024-05-07T11:49:00Z",
+    "2024-05-07T11:49:00+00:00",
+    "2024-05-07T11:49:00+0000"
+  ])
+  func decodesEveryUTCOffsetFormToSameInstant(input: String) throws {
+    // when
+    let date = try RFC3339Strategy.decode(input)
+
+    // then
+    // "ZZZZZ" still parses every offset form the old "Z" pattern accepted.
+    #expect(date == Date(timeIntervalSince1970: 1715082540))
+  }
+
+  @Test(arguments: [
+    "1996-12-19T16:39:57-08:00",
+    "1996-12-19T16:39:57-0800"
+  ])
+  func decodesNonUTCOffsetsWithOrWithoutColon(input: String) throws {
+    // when
+    let date = try RFC3339Strategy.decode(input)
+
+    // then
+    #expect(date == Date(timeIntervalSince1970: 851042397))
+  }
+}
+
+// MARK: - Decoding a malformed string
+
+extension RFC3339StrategyTests {
+
+  @Test
+  func throwsDataCorruptedErrorForMalformedString() {
+    // when / then
+    #expect {
+      try RFC3339Strategy.decode("not-a-valid-date")
+    } throws: { error in
+      guard case DecodingError.dataCorrupted = error else { return false }
+      return true
+    }
+  }
+}