BAEL-5928: Adds conversion b/w Epoch & LocalDateTime

core-java-modules/core-java-date-operations-3/src/main/java/com/baeldung/epochconversion/EpochToLocalDate.java

@@ -0,0 +1,24 @@
+package com.baeldung.epochconversion;
+
+import java.time.Instant;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+
+public class EpochToLocalDate {
+
+    public static void main(String[] args) {
+        long millisSinceEpoch = 2131242L;
+        ZoneId zoneId = ZoneId.of("Europe/Amsterdam"); // or: ZoneId.systemDefault();
+        // to get All Zone Ids available, we can use the function below
+        // Set<String> allZoneIds =  ZoneId.getAvailableZoneIds();
+        LocalDate date =
+            Instant.ofEpochMilli(millisSinceEpoch)
+                .atZone(zoneId)
+                .toLocalDate();
+        LocalDateTime time =
+            Instant.ofEpochMilli(millisSinceEpoch)
+                .atZone(zoneId)
+                .toLocalDateTime();
+    }
+}

core-java-modules/core-java-date-operations-3/src/main/java/com/baeldung/epochconversion/LocalDateToEpoch.java

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+package com.baeldung.epochconversion;
+
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+
+public class LocalDateToEpoch {
+
+    public static void main(String[] args) {
+        ZoneId zoneId = ZoneId.of("Europe/Amsterdam"); // or: ZoneId.systemDefault()
+        LocalDate date = LocalDate.now();
+        long epochMilliSecondsAtDate = date.atStartOfDay(zoneId).toInstant().toEpochMilli();
+
+        // epoch for time
+        LocalDateTime time = LocalDateTime.parse("2019-11-15T13:15:30");
+        long epochMilliSecondsAtTime = time.atZone(zoneId).toInstant().toEpochMilli();
+    }
+}

core-java-modules/core-java-date-operations-3/src/test/java/com/baeldung/epochconversion/EpochToLocalDateUnitTest.java

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+package com.baeldung.epochconversion;
+
+import org.junit.jupiter.api.Test;
+
+import java.time.Instant;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class EpochToLocalDateUnitTest {
+
+    @Test
+    void givenEpoch_thenComputeLocalDateCorrectly() {
+        ZoneId zoneId = ZoneId.systemDefault();
+        LocalDate expectedDate = LocalDate.of(1995, 4, 15);
+        long date = expectedDate.atStartOfDay(zoneId).toInstant().toEpochMilli();
+
+        LocalDate actualDate =
+            Instant.ofEpochMilli(date)
+                .atZone(zoneId)
+                .toLocalDate();
+        assertEquals(expectedDate, actualDate);
+    }
+
+    @Test
+    void givenEpoch_thenComputeLocalDateTimeCorrectly() {
+        ZoneId zoneId = ZoneId.systemDefault();
+        LocalDateTime expectedTime = LocalDateTime.parse("2019-11-15T13:15:30");
+        long time = expectedTime.atZone(zoneId).toInstant().toEpochMilli();
+
+        LocalDateTime actualTime =
+            Instant.ofEpochMilli(time)
+                .atZone(zoneId)
+                .toLocalDateTime();
+        assertEquals(expectedTime, actualTime);
+    }
+}

core-java-modules/core-java-date-operations-3/src/test/java/com/baeldung/epochconversion/LocalDateTimeToEpochUnitTest.java

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+package com.baeldung.epochconversion;
+
+import org.junit.jupiter.api.Test;
+
+import java.time.Instant;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class LocalDateTimeToEpochUnitTest {
+
+    @Test
+    void givenDate_thenComputeEpochCorrectly() {
+        ZoneId zoneId = ZoneId.of("Europe/Tallinn");
+        long expectedEpoch = LocalDate.now().toEpochDay();
+        LocalDateTime givenDate = Instant.ofEpochMilli(expectedEpoch)
+            .atZone(zoneId)
+            .toLocalDateTime();
+
+        long actualEpoch = givenDate.atZone(zoneId).toInstant().toEpochMilli();
+        assertEquals(expectedEpoch, actualEpoch);
+    }
+
+    @Test
+    void givenTime_thenComputeEpochCorrectly() {
+        ZoneId zoneId = ZoneId.of("Europe/Amsterdam");
+        long expectedEpoch = Instant.now().toEpochMilli();
+        LocalDateTime givenTime = Instant.ofEpochMilli(expectedEpoch)
+            .atZone(zoneId)
+            .toLocalDateTime();
+
+        long actualEpoch = givenTime.atZone(zoneId).toInstant().toEpochMilli();
+        assertEquals(expectedEpoch, actualEpoch);
+    }
+}