Show the user examples of when a cron tab list should actually run.

changelog.html

@@ -55,6 +55,8 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
+  <li class=rfe>
+    Cron-style trigger configuration will now display expected prior and subsequent run times.
   <li class=bug>
     Incorrect filtering of build queue and executors widgets after 1.514.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-20500">issue 20500</a>)

core/src/main/java/hudson/scheduler/CronTabList.java

@@ -24,10 +24,12 @@
 package hudson.scheduler;
 
 import antlr.ANTLRException;
-
 import java.util.Calendar;
 import java.util.Collection;
 import java.util.Vector;
+import javax.annotation.CheckForNull;
+import org.kohsuke.accmod.Restricted;
+import org.kohsuke.accmod.restrictions.NoExternalUse;
 
 /**
  * {@link CronTab} list (logically OR-ed).
@@ -89,4 +91,29 @@ public final class CronTabList {
         }
         return new CronTabList(r);
     }
+
+    @Restricted(NoExternalUse.class) // just for form validation
+    public @CheckForNull Calendar previous() {
+        Calendar nearest = null;
+        for (CronTab tab : tabs) {
+            Calendar scheduled = tab.floor(Calendar.getInstance());
+            if (nearest == null || nearest.before(scheduled)) {
+                nearest = scheduled;
+            }
+        }
+        return nearest;
+    }
+
+    @Restricted(NoExternalUse.class) // just for form validation
+    public @CheckForNull Calendar next() {
+        Calendar nearest = null;
+        for (CronTab tab : tabs) {
+            Calendar scheduled = tab.ceil(Calendar.getInstance());
+            if (nearest == null || nearest.after(scheduled)) {
+                nearest = scheduled;
+            }
+        }
+        return nearest;
+    }
+
 }

core/src/main/java/hudson/triggers/TimerTrigger.java

@@ -23,19 +23,21 @@
  */
 package hudson.triggers;
 
+import antlr.ANTLRException;
+import hudson.Extension;
 import static hudson.Util.fixNull;
 import hudson.model.BuildableItem;
 import hudson.model.Cause;
 import hudson.model.Item;
 import hudson.scheduler.CronTabList;
+import hudson.scheduler.Hash;
 import hudson.util.FormValidation;
-import hudson.Extension;
-
+import java.text.DateFormat;
+import java.util.Calendar;
+import org.kohsuke.stapler.AncestorInPath;
 import org.kohsuke.stapler.DataBoundConstructor;
 import org.kohsuke.stapler.QueryParameter;
 
-import antlr.ANTLRException;
-
 /**
  * {@link Trigger} that runs a job periodically.
  *
@@ -64,18 +66,26 @@ public class TimerTrigger extends Trigger<BuildableItem> {
         }
 
         // backward compatibility
-        public FormValidation doCheck(@QueryParameter String value) {
-            return doCheckSpec(value);
+        public FormValidation doCheck(@QueryParameter String value, @AncestorInPath Item item) {
+            return doCheckSpec(value, item);
         }
         
         /**
          * Performs syntax check.
          */
-        public FormValidation doCheckSpec(@QueryParameter String value) {
+        public FormValidation doCheckSpec(@QueryParameter String value, @AncestorInPath Item item) {
             try {
-                String msg = CronTabList.create(fixNull(value)).checkSanity();
+                CronTabList ctl = CronTabList.create(fixNull(value), item != null ? Hash.from(item.getFullName()) : null);
+                String msg = ctl.checkSanity();
                 if(msg!=null)   return FormValidation.warning(msg);
-                return FormValidation.ok();
+                Calendar prev = ctl.previous();
+                Calendar next = ctl.next();
+                if (prev != null && next != null) {
+                    DateFormat fmt = DateFormat.getDateTimeInstance(DateFormat.FULL, DateFormat.FULL);
+                    return FormValidation.ok(Messages.TimerTrigger_would_last_have_run_at_would_next_run_at(fmt.format(prev.getTime()), fmt.format(next.getTime())));
+                } else {
+                    return FormValidation.warning(Messages.TimerTrigger_no_schedules_so_will_never_run());
+                }
             } catch (ANTLRException e) {
                 if (value.trim().indexOf('\n')==-1 && value.contains("**"))
                     return FormValidation.error(Messages.TimerTrigger_MissingWhitespace());

core/src/main/resources/hudson/triggers/Messages.properties

@@ -26,5 +26,7 @@ SCMTrigger.BuildAction.DisplayName=Polling Log
 SCMTrigger.SCMTriggerCause.ShortDescription=Started by an SCM change
 TimerTrigger.DisplayName=Build periodically
 TimerTrigger.MissingWhitespace=You appear to be missing whitespace between * and *.
+TimerTrigger.no_schedules_so_will_never_run=No schedules so will never run
 TimerTrigger.TimerTriggerCause.ShortDescription=Started by timer
-Trigger.init=Initializing timer for triggers
\ No newline at end of file
+TimerTrigger.would_last_have_run_at_would_next_run_at=Would last have run at {0}; would next run at {1}.
+Trigger.init=Initializing timer for triggers