dhruwal

This plugin is useful when you want to know what date time & timezone is it on server where wordpress is hosted if you are in other timezone. 
Output looks like:
Server Date/Time: Sat 06th Dec,2008 11:17 am 
Server Timezone: America/Phoenix, (MST -07:00)
Version Change log:
v0.5 
Supports upto WordPress 3.7.1
v0.4 
Supports upto WordPress 3.6
v0.3 
Minimize css code 
Supports upto WordPress 3.5.1
v0.2 
Added description before date time 
Added timezone information 
Changed text color from red to green
v0.1 
Initial release

WP Server Date Time

This plugin shows the server local current date time & timezone in the upper right of your admin screen on every page.

Ravi Kiran Katha

WordPress has a site-level timezone setting configured from the admin back-end. All dates and times displayed on the front-end are typically displayed in this timezone.
This plugin enables the display of dates/times in the front-end using the browser’s timezone. This is done by overriding the WordPress’s ‘timezone_string’ and ‘gmt_offset’ options to match the user’s timezone for the duration of a particular request 
ONLY on the front-end interface.
<h4>Browser Timezone Detection</h4>
The browser’s timezone is detected using one of the following two methods
<ol>
<li><a href="http://pellepim.bitbucket.org/jstz/" rel="nofollow ugc">jsTimezoneDetect</a> (default)</li>
<li><a href="http://caniuse.com/geolocation" rel="nofollow ugc">HTML5 Geolocation API</a> and <a href="https://developers.google.com/maps/documentation/timezone" rel="nofollow ugc">Google Timezone API</a></li>
</ol>
jsTimezoneDetect (default) 
 This is the default timezone detection mechanism. Please refer to the script <a href="http://pellepim.bitbucket.org/jstz/" rel="nofollow ugc">home page</a> 
for the compatibility and other notes
HTML5 Geolocation API 
 Detects the current geo-location using <a href="http://caniuse.com/geolocation" rel="nofollow ugc">navigator.geolocation.getCurrentPosition</a>. The location thus detected will be sent to the <a href="https://developers.google.com/maps/documentation/timezone" rel="nofollow ugc">Google Timezone API</a> 
which returns the timezone id – similar to “Asia/Kolkata”.
This method requires you to
<ul>
<li>Enable the “Google Timezone API” option on the plugin options page</li>
<li>Specify a valid <a href="https://developers.google.com/maps/documentation/timezone/get-api-key" rel="nofollow ugc">Google API Key</a>
<ul>
<li>The key should be a browser key</li>
<li>The “Accepted HTTP Referrers” should include the domain name used by the hosting wordpress site or left blank to allow all referrers</li>
</ul>
</li>
</ul>
If the required options for the usage of Google Timezone API are not met, the detection mechanism falls back to the default one.
<h4>Post Timezone Detection</h4>
<ul>
<li>If the browser’s timezone id is not detected successfully, everything else behaves as if this plugin is not installed.</li>
<li>If the browser’s timezone id is detected, the following actions take place
<ul>
<li>A cookie will be set in the browser, based on the cookie options specified on the plugin options page</li>
<li>If a cookie already exists previously with the same id, no action is taken</li>
<li>If a cookie doesn’t already exist previously or exists with a different timezone id, then a window.reload is triggered to refresh the page so that the dates/times on the page are updated</li>
</ul>
</li>
</ul>
<h4>EventON Hook</h4>
This plugin provides a feature to hook into the EventON plugin. When enabled, all EventON dates/times are updated to reflect the browser’s timezone.
<h4>Shortcodes</h4>
This plugin defines the below shortcodes
<ol>
<li>[wp_user_timezone_id] => Returns the timezone id of the browser or null if not available.</li>
<li>[wp_user_timezone_offset] => Returns the timezone’s gmt offset in hours of the browser or null if not available.</li>
</ol>
<h4>Options</h4>
This plugin defines the below options to retrieve the original timezone_string and gmt_offset defined at the site level
<ol>
<li>get_option( ‘site_timezone_string’ ) => Returns the original timezone_string option for the site</li>
<li>get_option( ‘site_gmt_offset’ ) => Returns the original gmt_offset option for the site</li>
</ol>
<h4>Actions/Hooks</h4>
This plugin defines the below filters/hooks
<ol>
<li>apply_filters( ‘wp_user_timezone_id’, $tz_id ) => Allows you to override the timezone string determined by the plugin. This filter can be used to apply user’s timezone preference. If any plugin or wordpress core implements a feature to accept and store user’s preferred timezone, the ‘wp_user_timezone_id’ can be used to apply that timezone value to the front-end. Reference: <a href="https://core.trac.wordpress.org/ticket/18146" rel="nofollow ugc">Add user-level timezone setting</a>).</li>
</ol>
<h4>Admin Backend Dates/Times</h4>
Important: The admin back-end is not modified by this plugin. All dates/times displayed in the back-end are still in the original timezone specified by the WordPress General Settings.

WP User Timezone

Magnificent

WP User Timezone displays the front-end dates & times in the browser's local timezone without actually modifying your database.

WP Server Date Time vs WP User Timezone