BAEL-1074 move difference between dates from core-java to libraries (#2533)

* Different types of bean injection in Spring

* Difference between two dates in java

* Update README.md

* Simple clean of difference between dates

* Clean my test article

* Improve dates diff: for dates and datetimes

* Move difference between dates from core-java to libraries

libraries/pom.xml

@@ -498,6 +498,22 @@
             <artifactId>gt-swing</artifactId>
             <version>${geotools.version}</version>
         </dependency>
+        <dependency>
+            <groupId>com.darwinsys</groupId>
+            <artifactId>hirondelle-date4j</artifactId>
+            <version>RELEASE</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>joda-time</groupId>
+            <artifactId>joda-time</artifactId>
+            <version>${joda-time.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.darwinsys</groupId>
+            <artifactId>hirondelle-date4j</artifactId>
+            <version>${hirondelle-date4j.version}</version>
+        </dependency>
     </dependencies>
     <repositories>
         <repository>
@@ -564,5 +580,7 @@
         <streamex.version>0.6.5</streamex.version>
         <vavr.version>0.9.0</vavr.version>
         <geotools.version>15.2</geotools.version>
+        <joda-time.version>2.9.9</joda-time.version>
+        <hirondelle-date4j.version>1.5.1</hirondelle-date4j.version>
     </properties>
 </project>

libraries/src/test/java/com/baeldung/date/DateDiffUnitTest.java

@@ -0,0 +1,82 @@
+package com.baeldung.date;
+
+import org.junit.Test;
+
+import java.text.ParseException;
+import java.text.SimpleDateFormat;
+import java.time.Duration;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.util.Date;
+import java.util.Locale;
+import java.util.TimeZone;
+import java.util.concurrent.TimeUnit;
+
+import static org.junit.Assert.assertEquals;
+
+public class DateDiffUnitTest {
+
+    @Test
+    public void givenTwoDatesBeforeJava8_whenDifferentiating_thenWeGetSix() throws ParseException {
+        SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy", Locale.ENGLISH);
+        Date firstDate = sdf.parse("06/24/2017");
+        Date secondDate = sdf.parse("06/30/2017");
+
+        long diffInMillies = Math.abs(secondDate.getTime() - firstDate.getTime());
+        long diff = TimeUnit.DAYS.convert(diffInMillies, TimeUnit.MILLISECONDS);
+
+        assertEquals(diff, 6);
+    }
+
+    @Test
+    public void givenTwoDatesInJava8_whenDifferentiating_thenWeGetSix() {
+        LocalDate now = LocalDate.now();
+        LocalDate sixDaysBehind = now.minusDays(6);
+
+        Duration duration = Duration.between(now, sixDaysBehind);
+        long diff = Math.abs(duration.toDays());
+
+        assertEquals(diff, 6);
+    }
+
+    @Test
+    public void givenTwoDateTimesInJava8_whenDifferentiating_thenWeGetSix() {
+        LocalDateTime now = LocalDateTime.now();
+        LocalDateTime sixMinutesBehind = now.minusMinutes(6);
+
+        Duration duration = Duration.between(now, sixMinutesBehind);
+        long diff = Math.abs(duration.toMinutes());
+
+        assertEquals(diff, 6);
+    }
+
+    @Test
+    public void givenTwoDatesInJodaTime_whenDifferentiating_thenWeGetSix() {
+        org.joda.time.LocalDate now = org.joda.time.LocalDate.now();
+        org.joda.time.LocalDate sixDaysBehind = now.minusDays(6);
+
+        org.joda.time.Period period = new org.joda.time.Period(now, sixDaysBehind);
+        long diff = Math.abs(period.getDays());
+
+        assertEquals(diff, 6);
+    }
+
+    @Test
+    public void givenTwoDateTimesInJodaTime_whenDifferentiating_thenWeGetSix() {
+        org.joda.time.LocalDateTime now = org.joda.time.LocalDateTime.now();
+        org.joda.time.LocalDateTime sixMinutesBehind = now.minusMinutes(6);
+
+        org.joda.time.Period period = new org.joda.time.Period(now, sixMinutesBehind);
+        long diff = Math.abs(period.getDays());
+    }
+
+    @Test
+    public void givenTwoDatesInDate4j_whenDifferentiating_thenWeGetSix() {
+        hirondelle.date4j.DateTime now = hirondelle.date4j.DateTime.now(TimeZone.getDefault());
+        hirondelle.date4j.DateTime sixDaysBehind = now.minusDays(6);
+
+        long diff = Math.abs(now.numDaysFrom(sixDaysBehind));
+
+        assertEquals(diff, 6);
+    }
+}