This plugin allows you to setup a series of dates to count to or from in terms of years, months, weeks, days, hours, minutes, and/or seconds.

Noted That:

To install correctly this countdown-timer.zip.
Fisrt Download the countdown-timer.zip to your computer
Extract/Open countdown-timer.zip to Your Computer.
Then, Find readme.txt file inside countdown-timer.zip and Open readme.txt.
Now, Read the Requirements of this plugin. Which Wordpress Version and PHP Version are required to run this Plugin in Your Wordpress Site.
Then, Follow the the Tips Below.

Start the Tips:

1. First Download "countdown-timer.zip" Plugin to your Local Computer. (Click Download)

2. Then, Login to your "yourdomain.com/wp-admin" Dashboard.

3. Then, Click on "Plugins" + "Add New" from left sidemenu of Dashboard.

4. Now, Click on "Upload Plugin" button.

5. Now, Browse "countdown-timer.zip" Downloaded plugin from your computer, Where you downloaded countdown-timer.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Countdown Timer" folder is added on left sidemenu. Now, Click on "Countdown Timer" folder.

Noted that: If you do not see "Countdown Timer" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about Countdown Timer Configurtions and Settings or How to work "Countdown Timer" in your WordPress site.

After Activated Plugin According to Step-6 then,

Go to "Plugins" + "Installed Plugin" from Wordpress Admin Panel Leftside Menu.
or Direct go to: https://yourdomain/wp-admin/plugins.php
Then, Find "Countdown Timer" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is Countdown Timer
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide

Guide
Install the plugin using your preferred method of choice. Using the built-in WordPress installer is the preferred choice, but you can also do things the hard/manual way. Activate the timer. Events can be added and other settings modified in Settings > Countdown Timer in The Dashboard. Inserting countdown timers into your blog There are three places you can insert a countdown timer: Sidebar (using the widget) A post or page (using shortcodes) PHP (experts only) Adding to the sidebar Add the widget to the sidebar by going to Appearances > Widget in The Dashboard. Adding to a post or page If you want to insert the Countdown Timer into a page or post, you can use the following shortcodes to return all or a limited number of Countdown Timers, respectively: [fergcorp_cdt] [fergcorp_cdt max=##] Where ## is maximum number of results to be displayed, ordered by date. If you want to insert individual countdown timers, such as in posts or on pages, you can use the following shortcode: `[fergcorp_cdt_single date="ENTER_DATE_HERE"]` Example: Time until our wedding: [fergcorp_cdt_single date=”08 December 2012?] Where “ENTER_DATE_HERE” uses PHP’s strtotime function and will parse about any English textual date/time description (such as “08 December 2012”). A complete list of valid formats can be found on PHP’s Supported Date and Time Format page. PHP Countdown Timer also provides a PHP function designed to be accessed publicly so you can include it in elements of your site that may not be in The Loop. `fergcorp_countdownTimer(##)` Where ## is the maximum number of events you wish to be displayed. If $maxEvents is not given, the function will return all timers. Events are automatically sorted by date of occurrence. Note: You should also encapsulate calls with “function_exists” to prevent unintentional fatal errors if the plugin is deactivated. Limiting the number of countdown timers displayed If you’re using the widget, there is an option to set the maximum number of timers shown. If you are using the PHP code, $maxEvents is the maximum number of events you wish to be displayed. Events are automatically sorted by date of occurrence. Changing the font, size, and style using CSS You can makes changes to the appearance of Countdown Timer display using CSS. The following CSS classes are available: `fergcorp_countdownTimer_event_li` styles each List Item, each item encompasses one countdown event `fergcorp_countdownTimer_event_title` styles the title of the event `fergcorp_countdownTimer_event_linkTitle` styles the title of an event if it is linked `fergcorp_countdownTimer_event_time` styles the actual countdown timer `fergcorp_countdownTimer_timeUnit` styles the units (e.g. “25 days”) `fergcorp_countdownTimer_year` styles the year text `fergcorp_countdownTimer_month` styles the month text `fergcorp_countdownTimer_week` styles the week text `fergcorp_countdownTimer_day` styles the day text `fergcorp_countdownTimer_hour` styles the hour text `fergcorp_countdownTimer_minute` styles the minute text `fergcorp_countdownTimer_second` styles the second text

Install the plugin using your preferred method of choice. Using the built-in WordPress installer is the preferred choice, but you can also do things the hard/manual way.
Activate the timer.
Events can be added and other settings modified in Settings > Countdown Timer in The Dashboard.

Inserting countdown timers into your blog

There are three places you can insert a countdown timer:

Sidebar (using the widget)
A post or page (using shortcodes)
PHP (experts only)

Adding to the sidebar

Add the widget to the sidebar by going to Appearances > Widget in The Dashboard.

Adding to a post or page

If you want to insert the Countdown Timer into a page or post, you can use the following shortcodes to return all or a limited number of Countdown Timers, respectively:
[fergcorp_cdt]
[fergcorp_cdt max=##]

Where ## is maximum number of results to be displayed, ordered by date.

If you want to insert individual countdown timers, such as in posts or on pages, you can use the following shortcode:

[fergcorp_cdt_single date="ENTER_DATE_HERE"]

Example:
Time until our wedding:
[fergcorp_cdt_single date=”08 December 2012?]

Where “ENTER_DATE_HERE” uses PHP’s strtotime function and will parse about any English textual date/time description (such as “08 December 2012”). A complete list of valid formats can be found on PHP’s Supported Date and Time Format page.

PHP

Countdown Timer also provides a PHP function designed to be accessed publicly so you can include it in elements of your site that may not be in The Loop.

fergcorp_countdownTimer(##)

Where ## is the maximum number of events you wish to be displayed. If $maxEvents is not given, the function will return all timers.

Events are automatically sorted by date of occurrence.

Note: You should also encapsulate calls with “function_exists” to prevent unintentional fatal errors if the plugin is deactivated.

Limiting the number of countdown timers displayed

If you’re using the widget, there is an option to set the maximum number of timers shown. If you are using the PHP code, $maxEvents is the maximum number of events you wish to be displayed.

Events are automatically sorted by date of occurrence.

Changing the font, size, and style using CSS

You can makes changes to the appearance of Countdown Timer display using CSS.

The following CSS classes are available:

fergcorp_countdownTimer_event_li styles each List Item, each item encompasses one countdown event
fergcorp_countdownTimer_event_title styles the title of the event
fergcorp_countdownTimer_event_linkTitle styles the title of an event if it is linked
fergcorp_countdownTimer_event_time styles the actual countdown timer
fergcorp_countdownTimer_timeUnit styles the units (e.g. “25 days”)
fergcorp_countdownTimer_year styles the year text
fergcorp_countdownTimer_month styles the month text
fergcorp_countdownTimer_week styles the week text
fergcorp_countdownTimer_day styles the day text
fergcorp_countdownTimer_hour styles the hour text
fergcorp_countdownTimer_minute styles the minute text
fergcorp_countdownTimer_second styles the second text

days, weeks, hours, minutes, seconds, andor, months, years, setup, allows,

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Reffered: https://wordpress.org/