Verify reproducible build timestamps

src/it/project-build-outputTimestamp/verify.groovy

@@ -1 +1,10 @@
-assert new File( basedir, 'target/jdeb-it_1.0_all.deb' ).exists();
+import org.apache.commons.compress.archivers.ar.ArArchiveEntry
+import org.apache.commons.compress.archivers.ar.ArArchiveInputStream
+
+File deb = new File( basedir, 'target/jdeb-it_1.0_all.deb' )
+assert deb.exists();
+ArArchiveInputStream is = new ArArchiveInputStream(new FileInputStream(deb))
+ArArchiveEntry entry
+while ((entry = is.getNextEntry()) != null) {
+    assert entry.getLastModifiedDate().getTime() == 1724680800000L
+}