Count down timer

Hi all, I decided I wanted to make my son a countdown timer to show how long before his exams.

I'm new to Arduino so have a couple of questions, I have managed to get the days, hours, min and sec on the display but I cannot get it to start counting down so do I need a Real Time Clock Module or an LCD & Keypad Shield or is it just coding issue.

I was thinking os also having messages appear on school holidays as a remind him to revise. I'm not sure how you would add these messages being I have already given it a target date so would you set it up that on a set date a messages gets displayed or would i have to have to stop the target date the display the message then start a new target date, if that makes any sense. I will address this once i have resolved the first issue.

I'm using a Uno with a 16 x 2 LCD display.

Thank you for in advance for any helpful advice.

Some kind of date/time giving device will be needed. Using millis & Co will not be precise enough. For the LCD I am using the I2C supported ones. They are easy to connect and leaves a lot of I/O free.
But……

Attach Your code! Take a look at the advice given at the top about how to attach info to Your topic.
Preprogramming various messages and their interval of being displayed is quite simple.