Illustrative example. The dates and amounts below are provided for practice.
A countdown of zero should mean the occasion is here. It should not skip directly to the following year while you still have time to celebrate today.
Reproduce the zero
Enter month 10, day 9, and Calculate As Of October 9, 2027. Leave Birth Year blank. The next occurrence is October 9, 2027, the weekday is Saturday, and Days Until Birthday is 0. Next Birthday Countdown reads Today.
No whole calendar-day interval separates a date from itself. Under inclusive numbering, that same date is Day 1. This is why a display of zero days does not mean the birthday has already been missed.
If the recipient is in another time zone, confirm their local date before deciding how to phrase a greeting. Calculate As Of is a date you choose. The tool does not determine the recipient's location.
A result stays on screen until you calculate again. After midnight, refresh or recalculate with the new reference date. A yearly birthday record retains its month and day even when tomorrow's countdown begins pointing to next year's occurrence.