This is just a custom JQuery Datepicker that I followed from a tutorial online.
There was a time when I was developing a small web application, and it came to a phase where I wanted to add a Datepicker. I've scoured the internet to find something that I could just sprinkle into my simple PHP page. However, I couldn't find any decent ones that I can easily integrate with.
The easiest DatePicker that I can find is the same old JQuery Datepicker. But the JQuery Datepicker's default design is just not the one I'm looking for. So, I decided to find a way to customize it, and I stumbled upon a blog article that allowed me to reach my goal.
See: https://www.hongkiat.com/blog/jquery-ui-datepicker/
Just find the "implementation.html" somewhere in the source code directory, and it will show you how to implement it.