This is a very popular mod, particularly used in "100 Days" series on YouTube or by gamers who like to track their progress over the long term.
Here's how this type of mod (often called a Day Counter or Better Day Counter) generally works:
Automatic Display
The mod is designed to be discreet. Instead of having text cluttering your screen all the time, it only activates at key moments:
At sunrise: As soon as the day cycle begins again.
After a night's sleep: As soon as you get out of bed, the new day number is displayed prominently (often in the center of the screen or just above the inventory bar).
