Timestamp should be a property, and formatting should be done in the UI.

core/src/main/java/hudson/slaves/OfflineCause.java

@@ -29,10 +29,11 @@ import hudson.Functions;
 import hudson.model.Computer;
 import hudson.model.User;
 
-import org.acegisecurity.Authentication;
 import org.jvnet.localizer.Localizable;
 import org.kohsuke.stapler.export.ExportedBean;
 import org.kohsuke.stapler.export.Exported;
+
+import javax.annotation.Nonnull;
 import java.util.Date;
 
 /**
@@ -42,32 +43,51 @@ import java.util.Date;
  * <p>
  * {@link OfflineCause} must have <tt>cause.jelly</tt> that renders a cause
  * into HTML. This is used to tell users why the node is put offline.
- * This view should render a block element like DIV. 
+ * This view should render a block element like DIV.
  *
  * @author Kohsuke Kawaguchi
  * @since 1.320
  */
 @ExportedBean
 public abstract class OfflineCause {
+    protected final long timestamp = System.currentTimeMillis();
+
+    /**
+     * Timestamp in which the event happened.
+     *
+     * @since 1.612
+     */
+    @Exported
+    public long getTimestamp() {
+        return timestamp;
+    }
+
+    /**
+     * Same as {@link #getTimestamp()} but in a different type.
+     *
+     * @since 1.612
+     */
+    public final @Nonnull Date getTime() {
+        return new Date(timestamp);
+    }
+
     /**
      * {@link OfflineCause} that renders a static text,
      * but without any further UI.
      */
     public static class SimpleOfflineCause extends OfflineCause {
         public final Localizable description;
-        public final Date timestamp;
 
         /**
          * @since 1.571
          */
         protected SimpleOfflineCause(Localizable description) {
             this.description = description;
-            this.timestamp = new Date();
         }
 
         @Exported(name="description") @Override
         public String toString() {
-            return "[" + timestamp.toString() + "] " + description.toString();
+            return description.toString();
         }
     }
 

core/src/main/resources/hudson/slaves/OfflineCause/cause.jelly

@@ -23,6 +23,13 @@ THE SOFTWARE.
 -->
 
 <?jelly escape-by-default='true'?>
-<j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form">
-  <p class="warning"><j:out value="${it}" /></p>
+<j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form" xmlns:i="jelly:fmt">
+  <div class="warning">
+    <div class="timestamp">
+      <i:formatDate value="${it.time}" type="both" dateStyle="medium" timeStyle="medium"/>
+    </div>
+    <div class="message">
+      <j:out value="${it}" />
+    </div>
+  </div>
 </j:jelly>