Install the plugin from the ‘Plugins’ section in your dashboard (Go to Plugins > Add New > Search and search for Publish Date DatePicker).
Alternatively, you can download the plugin from the repository. Unzip it and upload it to the plugins folder of your WordPress installation (wp-content/plugins/ directory of your WordPress installation).
Activate it through the ‘Plugins’ section.
Does it cause to make site slow?
No. It only loads tiny css,image and js file in edit screen of post, page and custom post.
For anyone even just considering the installation of this plugin, please note that using this plugin's documentation link:
Access Plugin Documentation on http://freewptp.com/plugins/publish-date-datepicker/
Is redirected to a "casino" webpage (http://220.127.116.11/).
I don't see how anyone could trust the creator of this plugin. Reportedly completed fixes were not made (see below). At minimum, the creator's website has been hacked, or worse. My advise would be to use it at your own peril. I, with great haste, unintalled it.
My initial review was for 2 stars and was titled "Error was claimed to be fixed but was not."
Clicking the "now" option does not reset time to the current (valid) one. As a result, when updating a page, instead of right now your page won't be published until that future point in time (later in the day).
This error was reported over 4 years ago and at that time the plugin author claimed that this error was fixed. It wasn't. It still occurs here in the year 2021.
The "now" button is a great feature, if only it worked.
This error still exists, but pales compared to the newer issue I mention.