LocalDateTimes > From Zone to Zone util methods

src/main/kotlin/javatimefun/localdatetime/extensions/LocalDateTimeMutatingExtensions.kt

@@ -1,8 +1,10 @@
 package javatimefun.localdatetime.extensions
 
+import javatimefun.ZoneIds
 import java.time.DayOfWeek
 import java.time.LocalDateTime
 import java.time.LocalTime
+import java.time.ZoneId
 
 fun LocalDateTime.atStartOfDay(): LocalDateTime = this.withLocalTime(LocalTime.MIN)
 
@@ -40,4 +42,18 @@ fun LocalDateTime.getNext(dayOfWeek: DayOfWeek, countingInThisDay: Boolean = fal
         nextLocalDate = nextLocalDate.plusDays(1)
     }
     return nextLocalDate
-}
+}
+
+fun LocalDateTime.fromZoneToZone(fromZoneId: ZoneId, toZoneId: ZoneId): LocalDateTime {
+    if (fromZoneId == toZoneId) return this
+    return this
+        .atZone(fromZoneId)
+        .withZoneSameInstant(toZoneId)
+        .toLocalDateTime()
+}
+
+fun LocalDateTime.fromUtcToZone(toZoneId: ZoneId): LocalDateTime =
+    this.fromZoneToZone(ZoneIds.UTC, toZoneId)
+
+fun LocalDateTime.fromZoneToUtc(fromZoneId: ZoneId): LocalDateTime =
+    this.fromZoneToZone(fromZoneId, ZoneIds.UTC)

src/test/kotlin/localdatetime/LocalDateTimeMutatingTest.kt

@@ -0,0 +1,92 @@
+package localdatetime
+
+import javatimefun.ZoneIds
+import javatimefun.localdatetime.extensions.fromUtcToZone
+import javatimefun.localdatetime.extensions.fromZoneToUtc
+import javatimefun.localdatetime.extensions.fromZoneToZone
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+import java.time.LocalDateTime
+
+class LocalDateTimeMutatingTest {
+
+    @Test
+    fun `fromZoneToZone returns same time if zones are identical`() {
+        val time = LocalDateTime.of(2025, 6, 15, 12, 0)
+        val zone = ZoneIds.AFRICA_CAIRO
+
+        val converted = time.fromZoneToZone(zone, zone)
+        assertEquals(time, converted, "When fromZoneId and toZoneId are the same, time should not change")
+    }
+
+    @Test
+    fun `fromZoneToZone converts correctly between different zones`() {
+        val timeInTokyo = LocalDateTime.of(2025, 6, 15, 12, 0) // Noon in Tokyo
+        val fromZone = ZoneIds.AFRICA_CAIRO
+        val toZone = ZoneIds.AMERICA_ARGENTINA_Catamarca
+
+        val converted = timeInTokyo.fromZoneToZone(fromZone, toZone)
+
+        // Convert via ZonedDateTime for expected result
+        val expected = timeInTokyo.atZone(fromZone)
+            .withZoneSameInstant(toZone)
+            .toLocalDateTime()
+
+        assertEquals(expected, converted)
+    }
+
+    @Test
+    fun `fromUtcToZone converts correctly`() {
+        val timeUtc = LocalDateTime.of(2025, 6, 15, 12, 0)
+        val toZone = ZoneIds.AFRICA_CAIRO
+
+        val converted = timeUtc.fromUtcToZone(toZone)
+        val expected = timeUtc.atZone(ZoneIds.UTC)
+            .withZoneSameInstant(toZone)
+            .toLocalDateTime()
+
+        assertEquals(expected, converted)
+    }
+
+    @Test
+    fun `fromZoneToUtc converts correctly`() {
+        val timeInTokyo = LocalDateTime.of(2025, 6, 15, 12, 0)
+        val fromZone = ZoneIds.AFRICA_CAIRO
+
+        val converted = timeInTokyo.fromZoneToUtc(fromZone)
+        val expected = timeInTokyo.atZone(fromZone)
+            .withZoneSameInstant(ZoneIds.UTC)
+            .toLocalDateTime()
+
+        assertEquals(expected, converted)
+    }
+
+    @Test
+    fun `conversion handles DST transition correctly`() {
+        // Example: US Eastern DST ends on Nov 2, 2014 at 2:00 am
+        val fromZone = ZoneIds.AMERICA_NEW_YORK
+        val toZone = ZoneIds.EUROPE_LONDON
+
+        // 1:30 AM EDT (before DST ends)
+        val timeBeforeDSTEnd = LocalDateTime.of(2014, 11, 2, 1, 30)
+
+        val convertedBefore = timeBeforeDSTEnd.fromZoneToZone(fromZone, toZone)
+
+        val expectedBefore = timeBeforeDSTEnd.atZone(fromZone)
+            .withZoneSameInstant(toZone)
+            .toLocalDateTime()
+
+        assertEquals(expectedBefore, convertedBefore)
+
+        // 2:30 AM EST (after DST ends - clocks moved back 1 hour)
+        val timeAfterDSTEnd = LocalDateTime.of(2014, 11, 2, 2, 30)
+
+        val convertedAfter = timeAfterDSTEnd.fromZoneToZone(fromZone, toZone)
+
+        val expectedAfter = timeAfterDSTEnd.atZone(fromZone)
+            .withZoneSameInstant(toZone)
+            .toLocalDateTime()
+
+        assertEquals(expectedAfter, convertedAfter)
+    }
+}