Why a Countdown Timer Showed the Wrong Number of Days
The most common causes are a mismatch between the time zone the target date was set in and the device's current time zone, confusion between an inclusive and exclusive day count (as covered in the days-between-dates guide), and, less commonly, a device clock that's drifted out of sync with actual current time.
A countdown showing a number that feels wrong is almost always explainable by one of these specific causes rather than a calculation error in the tool itself.
Time zone mismatches
If a target date and time was set with one time zone in mind but the countdown is being viewed from a device in a different time zone, the displayed remaining time can be off by however many hours separate the two zones — worth double-checking, especially for a countdown to an event happening in a different location than where it's being viewed from.
Inclusive vs. exclusive counting
Whether "today" counts as one of the remaining days, or only fully elapsed days are counted, is the same ambiguity covered in the days-between-dates guide — a countdown showing one fewer (or more) day than expected is often simply a different, equally valid counting convention.
Device clock drift
Less commonly, a device's system clock can drift slightly out of sync with actual current time, especially if it hasn't synced with a time server recently — checking the device's current time against a reliable external source rules this out as a cause.