track uptime

core/src/main/java/jenkins/model/Uptime.java

+package jenkins.model;
+
+import hudson.Extension;
+import hudson.init.InitMilestone;
+import hudson.init.Initializer;
+
+/**
+ * Keeps track of the uptime of Jenkins.
+ *
+ * @author Kohsuke Kawaguchi
+ * @since 1.538
+ */
+@Extension
+public class Uptime {
+    private long startTime;
+
+    /**
+     * Timestamp in which Jenkins became fully up and running.
+     */
+    public long getStartTime() {
+        return startTime;
+    }
+
+    public long getUptime() {
+        return System.currentTimeMillis()-startTime;
+    }
+
+    @Initializer(after=InitMilestone.JOB_LOADED)
+    public static void init() {
+        Jenkins.getInstance().getInjector().getInstance(Uptime.class).startTime = System.currentTimeMillis();
+    }
+}