Use the "UTC" identifier for the TimeZone.UTC time zone

Fixes #474

Author: dkhalanskyjb

core/common/src/TimeZone.kt

@@ -75,7 +75,7 @@ public expect open class TimeZone {
         /**
          * Returns the time zone with the fixed UTC+0 offset.
          *
-         * The [id] of this time zone is `"Z"`.
+         * The [id] of this time zone is `"UTC"`.
          *
          * @sample kotlinx.datetime.test.samples.TimeZoneSamples.utc
          */

core/common/test/TimeZoneTest.kt

@@ -16,7 +16,7 @@ class TimeZoneTest {
     fun utc() {
         val utc: FixedOffsetTimeZone = TimeZone.UTC
         println(utc)
-        assertEquals("Z", utc.id)
+        assertEquals("UTC", utc.id)
         assertEquals(UtcOffset.ZERO, utc.offset)
         assertEquals(0, utc.offset.totalSeconds)
         assertEquals(utc.offset, utc.offsetAt(Clock.System.now()))

core/commonKotlin/src/TimeZone.kt

@@ -24,14 +24,17 @@ public actual open class TimeZone internal constructor() {
             return zone ?: of(name)
         }
 
-        public actual val UTC: FixedOffsetTimeZone = UtcOffset.ZERO.asTimeZone()
+        public actual val UTC: FixedOffsetTimeZone = FixedOffsetTimeZone(UtcOffset.ZERO, "UTC")
 
         // org.threeten.bp.ZoneId#of(java.lang.String)
         public actual fun of(zoneId: String): TimeZone {
             // TODO: normalize aliases?
-            if (zoneId == "Z") {
+            if (zoneId == "UTC") {
                 return UTC
             }
+            if (zoneId == "Z") {
+                return UtcOffset.ZERO.asTimeZone()
+            }
             if (zoneId == "SYSTEM") {
                 return currentSystemDefault()
             }

core/jvm/src/TimeZoneJvm.kt

@@ -32,7 +32,8 @@ public actual open class TimeZone internal constructor(internal val zoneId: Zone
 
     public actual companion object {
         public actual fun currentSystemDefault(): TimeZone = ofZone(ZoneId.systemDefault())
-        public actual val UTC: FixedOffsetTimeZone = UtcOffset(jtZoneOffset.UTC).asTimeZone()
+        public actual val UTC: FixedOffsetTimeZone =
+            FixedOffsetTimeZone(UtcOffset.ZERO, ZoneId.of("UTC"))
 
         public actual fun of(zoneId: String): TimeZone = try {
             ofZone(ZoneId.of(zoneId))

core/jvm/test/ConvertersTest.kt

@@ -147,6 +147,7 @@ class ConvertersTest {
         }
 
         test("Z")
+        test("UTC")
         test("Etc/UTC")
         test("+00")
         test("+0000")

core/linux/src/internal/TimeZoneNative.kt

@@ -48,7 +48,7 @@ internal actual fun currentSystemDefaultZone(): Pair<String, TimeZone?> {
     // According to https://www.man7.org/linux/man-pages/man5/localtime.5.html, UTC is used when /etc/localtime is missing.
     // If /etc/localtime exists but isn't a symlink, we check if it's a copy of a timezone file by examining /etc/timezone
     // (which is a Debian-specific approach used in older distributions).
-    val zonePath = currentSystemTimeZonePath ?: return "Z" to null
+    val zonePath = currentSystemTimeZonePath ?: return "UTC" to null
 
     zonePath.splitTimeZonePath()?.second?.toString()?.let { zoneId ->
         return zoneId to null

js-without-timezones/common/test/TimezonesWithoutDatabaseTest.kt

@@ -29,7 +29,7 @@ class TimezonesWithoutDatabaseTest {
     fun utc() {
         val utc: FixedOffsetTimeZone = TimeZone.UTC
         println(utc)
-        assertEquals("Z", utc.id)
+        assertEquals("UTC", utc.id)
         assertEquals(UtcOffset.ZERO, utc.offset)
         assertEquals(0, utc.offset.totalSeconds)
         assertEquals(utc.offset, utc.offsetAt(Clock.System.now()))