diff --git a/nbbuild/antsrc/org/netbeans/nbbuild/JarWithModuleAttributes.java b/nbbuild/antsrc/org/netbeans/nbbuild/JarWithModuleAttributes.java
--- a/nbbuild/antsrc/org/netbeans/nbbuild/JarWithModuleAttributes.java
+++ b/nbbuild/antsrc/org/netbeans/nbbuild/JarWithModuleAttributes.java
@@ -263,6 +263,9 @@
             if (buildNumber == null) {
                 throw new BuildException("Must have defined 'buildnumber'", getLocation());
             }
+            if (buildNumber.matches("\\d+")) {
+                throw new BuildException("'buildnumber' must not be set to something that looks like an integer: " + buildNumber, getLocation());
+            }
             String attrToAdd = implVers != null ? "OpenIDE-Module-Build-Version" : "OpenIDE-Module-Implementation-Version";
             added.addConfiguredAttribute(new Manifest.Attribute(attrToAdd, buildNumber));
             // If spec.version.base is defined, use it, after tacking on any numeric impl deps (sorted by CNB of the dep for stability),
diff --git a/nbbuild/templates/common.xml b/nbbuild/templates/common.xml
--- a/nbbuild/templates/common.xml
+++ b/nbbuild/templates/common.xml
@@ -229,7 +229,7 @@
     <target name="jar-prep" depends="init">
         <mkdir dir="${cluster}/${module.jar.dir}"/>
         <tstamp>
-            <format property="buildnumber" pattern="yyMMdd" timezone="UTC"/>
+            <format property="buildnumber" pattern="yyyy-MM-dd" timezone="UTC"/>
         </tstamp>
     </target>