How Old Am I calculates and displays ages in several formats.

Noted That:

To install correctly this how-old-am-i.zip.
Fisrt Download the how-old-am-i.zip to your computer
Extract/Open how-old-am-i.zip to Your Computer.
Then, Find readme.txt file inside how-old-am-i.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 "how-old-am-i.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 "how-old-am-i.zip" Downloaded plugin from your computer, Where you downloaded how-old-am-i.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "How Old Am I" folder is added on left sidemenu. Now, Click on "How Old Am I" folder.

Noted that: If you do not see "How Old Am I" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about How Old Am I Configurtions and Settings or How to work "How Old Am I" 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 "How Old Am I" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is How Old Am I
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide
How to install Upload the extracted archive to wp-content/plugins/ or upload the archive from the admin dashboard (Plugins -> Add New -> Upload). Activate the plugin via the Plugins menu How to configure Open the plugin settings page (Settings -> How Old Am I). Enter your birth date. Select a display format. Select how to deal with negative ages. How to use Select your date of birth and enter the shortcode [how-old-am-i] in any post or page. The following attributes are available to be used in the shortcode (the attributes can be combined): on — takes as an argument either a date (format: YYYY-MM-DD) and overrides the current date, or the word “post” and uses the date & time of the post to override the current date bday — takes as an argument either a date (format: YYYY-MM-DD) and overrides the birth date set on the plugin’s settings, or the word “post” and uses the date & time of the post to override the birth date Some examples using the attributes: [how-old-am-i on=”2013-03-01?] — displays the age as it was on the 1st of March, 2013 [how-old-am-i on=”post”] — displays the age as it was on the date the post was published on [how-old-am-i bday=”1980-02-22?] — displays the age using as a birth date the 22nd of February, 1980 [how-old-am-i bday=”post”] — displays the age using as a birth date the date that the post was published on [how-old-am-i on=”2013-03-01? bday=”1980-02-22?] — displays the age of a person born on the 22nd of February, 1980, as it was on the 1st of March, 2013, ignoring both the birth date set in the plugin’s setting, and the current date [how-old-am-i on=”post” bday=”1980-02-22?] — the same example as before, using the publish date of the post as the current date [how-old-am-i on=”2013-03-01? bday=”post”] — the same example as before, using the publish date of the post as the birth date

Guide

How to install

Upload the extracted archive to wp-content/plugins/ or upload the archive from the admin dashboard (Plugins -> Add New -> Upload).
Activate the plugin via the Plugins menu

How to configure

Open the plugin settings page (Settings -> How Old Am I).
Enter your birth date.
Select a display format.
Select how to deal with negative ages.

How to use

Select your date of birth and enter the shortcode [how-old-am-i] in any post or page.

The following attributes are available to be used in the shortcode (the attributes can be combined):

on — takes as an argument either a date (format: YYYY-MM-DD) and overrides the current date, or the word “post” and uses the date & time of the post to override the current date
bday — takes as an argument either a date (format: YYYY-MM-DD) and overrides the birth date set on the plugin’s settings, or the word “post” and uses the date & time of the post to override the birth date

Some examples using the attributes:

[how-old-am-i on=”2013-03-01?] — displays the age as it was on the 1st of March, 2013
[how-old-am-i on=”post”] — displays the age as it was on the date the post was published on
[how-old-am-i bday=”1980-02-22?] — displays the age using as a birth date the 22nd of February, 1980
[how-old-am-i bday=”post”] — displays the age using as a birth date the date that the post was published on
[how-old-am-i on=”2013-03-01? bday=”1980-02-22?] — displays the age of a person born on the 22nd of February, 1980, as it was on the 1st of March, 2013, ignoring both the birth date set in the plugin’s setting, and the current date
[how-old-am-i on=”post” bday=”1980-02-22?] — the same example as before, using the publish date of the post as the current date
[how-old-am-i on=”2013-03-01? bday=”post”] — the same example as before, using the publish date of the post as the birth date

formats, several, ages, displays, calculates,

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Reffered: https://wordpress.org/