This plugin adds a maintenance countdown page to wordpress.

Phil Niehus

The Maintenance Countdown plugin adds a maintenance countdown page to your wordpress installation. This gives you the 
opportunity to tell your users what you are doing and how long they need to wait until your site is back online. 
Users with the ‘edit_themes’ capability are still able to access the site when maintenance is enabled.
Features:
<ul>
<li>Enable/disable maintenance mode</li>
<li>Page title, headline, maintenance information and check back message are customizable through the dashboard</li>
<li>The end date and time for the countdown can be set through the settings page on the dashboard</li>
<li>The countdown starts counting upwards if the end time is exceeded</li>
<li>Includes 2 different designs</li>
<li>Has an option to use custom css, for more information take a look at the faq</li>
</ul>

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