Bael 9349 adding milliseconds to java date (#18737)

* [BAEL-9349] Add main program and unit tests

* [BAEL-9349] - minor fix

Author: MBuczkowski2025

core-java-modules/core-java-8-datetime-4/src/main/java/com/baeldung/instantandlong/InstantAndLong.java

@@ -0,0 +1,34 @@
+package com.baeldung.instantandlong;
+
+import java.time.Instant;
+import java.time.temporal.ChronoUnit;
+
+public class InstantAndLong {
+
+    public static void main(String[] args) {
+        long nowLong = Instant.now().toEpochMilli();
+
+        long someDayLong = 1_753_610_399_076L;
+        Instant someDay = Instant.ofEpochMilli(someDayLong);
+
+        long expirationPeriod = 2_592_000_000L; // 30 days in milliseconds
+        Instant now = Instant.now();
+        Instant expirationTime = now.plus(expirationPeriod, ChronoUnit.MILLIS);
+
+        expirationTime = Instant.now().plusMillis(2_592_000_000L);
+
+        Instant aDayAgo = now.minus(86_400_000L, ChronoUnit.MILLIS);
+
+        aDayAgo = now.plus(-86_400_000L, ChronoUnit.MILLIS);
+        
+        expirationPeriod = 30    // number of days
+                         * 24    // hours in one day
+                         * 3600  // seconds in one hour
+                         * 1000L;// from seconds to milliseconds
+
+        nowLong = Instant.now().toEpochMilli();
+
+        long expirationTimeLong = nowLong + expirationPeriod;
+    }
+
+}

core-java-modules/core-java-8-datetime-4/src/test/java/com/baeldung/instantandlong/InstantAndLongUnitTest.java

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+package com.baeldung.instantandlong;
+
+
+import org.junit.Test;
+import static org.junit.Assert.assertEquals;
+
+import java.time.Instant;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import java.time.ZonedDateTime;
+import java.time.format.DateTimeFormatter;
+import java.time.temporal.ChronoUnit;
+import java.util.Locale;
+
+public class InstantAndLongUnitTest {
+
+    private long someDayLong = 1_753_610_399_076L;
+    private long oneDayLong = 86_400_000L;
+    private String stringDate = "2025-01-30T17:33:21";
+    //2025.01.30 17:33:21 in milliseconds, Java epoch
+    private long dayInMillis = ((((2025 - 1970) * 365 + 29 + 14) * 24 // days, including an additional day for each of the 14 leap years
+                                + 17) * 3600  //to seconds
+                                + 33*60 + 21) //add minutes and seconds
+                                * 1000L;      //to milliseconds
+
+    @Test
+    public void whenPlusMillis_thenNextDay() {
+        Instant someDay = Instant.ofEpochMilli(someDayLong);
+        Instant nextDay = someDay.plusMillis(oneDayLong);
+
+        assertEquals(nextDay.toEpochMilli(), someDayLong + oneDayLong);
+    }
+
+    @Test
+    public void whenPlus_thenNextDay() {
+        Instant someDay = Instant.ofEpochMilli(someDayLong);
+        Instant nextDay = someDay.plus(oneDayLong, ChronoUnit.MILLIS);
+
+        assertEquals(nextDay.toEpochMilli(), someDayLong + oneDayLong);
+    }
+
+    @Test
+    public void whenMinusMillis_thenPreviousDay() {
+        Instant someDay = Instant.ofEpochMilli(someDayLong);
+        Instant previousDay = someDay.minusMillis(oneDayLong);
+
+        assertEquals(previousDay.toEpochMilli(), someDayLong - oneDayLong);
+    }
+
+    @Test
+    public void whenMinus_thenPreviousDay() {
+        Instant someDay = Instant.ofEpochMilli(someDayLong);
+        Instant previousDay = someDay.minus(oneDayLong, ChronoUnit.MILLIS);
+
+        assertEquals(previousDay.toEpochMilli(), someDayLong - oneDayLong);
+    }
+
+    @Test
+    public void whenToEpochMilli_thenDaysInMillis() {
+        LocalDateTime dateTime = LocalDateTime.parse(stringDate, DateTimeFormatter.ISO_LOCAL_DATE_TIME);
+        ZonedDateTime zonedDateTime = dateTime.atZone(ZoneId.of("UTC"));
+        Instant instant = zonedDateTime.toInstant();
+
+        assertEquals(instant.toEpochMilli(), dayInMillis);
+    }
+
+    @Test
+    public void whenOfEpochMilli_thenDateTimeAsInstant() {
+        LocalDateTime dateTime = LocalDateTime.parse(stringDate, DateTimeFormatter.ISO_LOCAL_DATE_TIME);
+        ZonedDateTime zonedDateTime = dateTime.atZone(ZoneId.of("UTC"));
+        Instant instant = zonedDateTime.toInstant();
+
+        assertEquals(Instant.ofEpochMilli(dayInMillis), instant);
+    }
+
+}
+