Added exercise gigasecond (see #3)

config.json

@@ -14,7 +14,8 @@
     "meetup",
     "phone-number",
     "grade-school",
-    "space-age"
+    "space-age",
+    "gigasecond"
   ],
   "deprecated": [
   ],

gigasecond/build.gradle

@@ -0,0 +1,11 @@
+apply plugin: "java"
+apply plugin: "eclipse"
+apply plugin: "idea"
+
+repositories {
+  mavenCentral()
+}
+
+dependencies {
+  testCompile "junit:junit:4.10"
+}

gigasecond/example.java

@@ -0,0 +1,3 @@
+public class Gigasecond {
+
+}

gigasecond/src/main/java/Gigasecond.java

@@ -0,0 +1,19 @@
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+
+public class Gigasecond {
+
+    private LocalDateTime birthDateTime;
+
+    public Gigasecond(LocalDateTime birthDateTime) {
+        this.birthDateTime = birthDateTime;
+    }
+
+    public Gigasecond(LocalDate birthDate) {
+        this.birthDateTime = birthDate.atTime(0, 0);
+    }
+
+    public LocalDateTime Date() {
+        return birthDateTime.plusSeconds(1000000000);
+    }
+}

gigasecond/src/test/java/GigasecondTest.java

@@ -0,0 +1,45 @@
+import org.junit.Test;
+
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.Month;
+
+import static org.junit.Assert.assertEquals;
+
+public class GigasecondTest {
+
+    @Test
+    public void modernTime() {
+        Gigasecond gigaSecond = new Gigasecond(LocalDate.of(2011, Month.APRIL, 25));
+
+        assertEquals(gigaSecond.Date(), LocalDateTime.of(2043, Month.JANUARY, 1, 1, 46, 40));
+    }
+
+    @Test
+    public void afterEpochTime() {
+        Gigasecond gigaSecond = new Gigasecond(LocalDate.of(1977, Month.JUNE, 13));
+
+        assertEquals(gigaSecond.Date(), LocalDateTime.of(2009, Month.FEBRUARY, 19, 1, 46, 40));
+    }
+
+    @Test
+    public void beforeEpochTime() {
+        Gigasecond gigaSecond = new Gigasecond(LocalDate.of(1959, Month.JULY, 19));
+
+        assertEquals(gigaSecond.Date(), LocalDateTime.of(1991, Month.MARCH, 27, 1, 46, 40));
+    }
+
+    @Test
+    public void withFullTimeSpecified() {
+        Gigasecond gigaSecond = new Gigasecond(LocalDateTime.of(2015, Month.JANUARY, 24, 22, 0, 0));
+
+        assertEquals(gigaSecond.Date(), LocalDateTime.of(2046, Month.OCTOBER, 2, 23, 46, 40));
+    }
+
+    @Test
+    public void withFullTimeSpecifiedAndDayRollover() {
+        Gigasecond gigaSecond = new Gigasecond(LocalDateTime.of(2015, Month.JANUARY, 24, 23, 59, 59));
+
+        assertEquals(gigaSecond.Date(), LocalDateTime.of(2046, Month.OCTOBER, 3, 1, 46, 39));
+    }
+}