diff --git a/HISTORY.md b/HISTORY.md
index 3ecfe9b0..8ee7ce20 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -19,6 +19,8 @@ http://visjs.org
   `.vis.timeline .item.range { overflow: visible; }` instead of
   `.vis.timeline .item.range .content { overflow: visible; }`.
   See example 18_range_overflow.html.
+- Fixed invalid css names for time axis grid, renamed hours class names from
+  `4-8h` to `h4-h8`.
 
 
 ## not yet released, version 3.9.2-SNAPSHOT
diff --git a/docs/timeline.html b/docs/timeline.html
index 59eec23e..96a7c333 100644
--- a/docs/timeline.html
+++ b/docs/timeline.html
@@ -1382,10 +1382,10 @@ To load a locale into the Timeline not supported by default, one can add a new l
     <td>Current date</td><td><code>today</code>, <code>tomorrow</code>, <code>yesterday</code>, <code>current-week</code>, <code>current-month</code>, <code>current-year</code></td>
   </tr>
   <tr>
-    <td>Hours</td><td><code>0h</code>, <code>1h</code>, ..., <code>23h</code></td>
+    <td>Hours</td><td><code>h0</code>, <code>h1</code>, ..., <code>h23</code></td>
   </tr>
   <tr>
-    <td>Grouped hours</td><td><code>0-4h</code> to <code>20-24h</code></td>
+    <td>Grouped hours</td><td><code>h0-h4</code> to <code>h20-h24</code></td>
   </tr>
   <tr>
     <td>Weekday</td><td><code>monday</code>, <code>tuesday</code>, <code>wednesday</code>, <code>thursday</code>, <code>friday</code>, <code>saturday</code>, <code>sunday</code></td>
diff --git a/lib/timeline/TimeStep.js b/lib/timeline/TimeStep.js
index 8d8bc7cb..5313ace1 100644
--- a/lib/timeline/TimeStep.js
+++ b/lib/timeline/TimeStep.js
@@ -576,9 +576,9 @@ TimeStep.prototype.getClassName = function() {
     case 'hour':
       var hours = date.hours();
       if (this.step == 4) {
-        hours = hours + '-' + (hours + 4);
+        hours = hours + '-h' + (hours + 4);
       }
-      return hours + 'h' + today(date) + even(date.hours());
+      return 'h' + hours + today(date) + even(date.hours());
 
     case 'weekday':
       return date.format('dddd').toLowerCase() +