Seasonal
The number is a picture of today's count. A wallpaper can't change by itself, so the Till widget is what keeps counting.
Keep this on your Home Screen
Till puts a live countdown widget on your iPhone Home Screen, and you can share it with your person so you both see the same number.
iOS 17 or laterQuestions about the wallpaper
Does the number update by itself?
No. A wallpaper is a still image, so it shows the days left on the day you made it. Make a new one when you want to refresh it, or use the Till widget, which counts on its own.
How do I set it as my lock screen?
On iPhone, open the image in Photos, tap Share and choose Use as Wallpaper. On Android, open it in Gallery and choose Set as wallpaper.
Is anything uploaded?
No. The image is drawn on your device and downloaded straight from your browser.