Use project.build.outputTimestamp as release date

This should make building the distributions reproducible. The timestamp
is updated automatically during the release.
diff --git a/modules/distribution/pom.xml b/modules/distribution/pom.xml
index 6bf8280..2d571fa 100755
--- a/modules/distribution/pom.xml
+++ b/modules/distribution/pom.xml
@@ -377,9 +377,10 @@
                         <configuration>
                             <scripts>
                                 <script>
-                                    import java.util.Date 
-                                    import java.text.MessageFormat 
-                                    project.properties['buildTimestamp'] = MessageFormat.format("{0,date,dd-MM-yyyy}", new Date())
+                                    import java.time.*
+                                    import java.time.format.*
+
+                                    project.properties['buildTimestamp'] = DateTimeFormatter.ofPattern("dd-MM-yyyy").withZone(ZoneId.of("Z")).format(OffsetDateTime.parse(project.properties['project.build.outputTimestamp']).toInstant())
                                     project.properties['release_version'] = project.version.replaceAll("-SNAPSHOT", "")
                                 </script>
                             </scripts>
diff --git a/pom.xml b/pom.xml
index 42f2417..d6142be 100644
--- a/pom.xml
+++ b/pom.xml
@@ -1445,9 +1445,10 @@
                         <configuration>
                             <scripts>
                                 <script>
-                                    import java.util.Date
-                                    import java.text.MessageFormat
-                                    project.properties['release_date'] = MessageFormat.format("{0,date,MMMMM dd, yyyy}", new Date())
+                                    import java.time.*
+                                    import java.time.format.*
+
+                                    project.properties['release_date'] = DateTimeFormatter.ofPattern("MMMM dd, yyyy").withZone(ZoneId.of("Z")).format(OffsetDateTime.parse(project.properties['project.build.outputTimestamp']).toInstant())
                                     project.properties['release_version'] = project.version.replaceAll("-SNAPSHOT", "")
                                     project.properties['skipSiteSite'] = String.valueOf(!new File(project.basedir, 'src/site/site.xml').exists())
                                 </script>