Full Days vs. Calendar Days
The timer shows complete 24-hour periods, followed by the remaining hours, minutes, and seconds. On December 24 it can show 0 days with several hours still remaining. That means Christmas has not arrived yet.
Calendar days count how many times your local date changes before December 25. For that view, use the sleeps countdown: December 24 is one sleep before Christmas morning.
Why Countries Can Show Different Counts
Christmas starts at midnight in your chosen timezone. Sydney reaches December 25 before London, Toronto, or Los Angeles. This page uses your device timezone; the main timer also lets you choose a fixed timezone in Settings.
The countdown follows your device clock. If the time looks wrong, check your device date, time, and timezone. It targets Christmas Day itself, rather than school holidays or store closing times.