A birthday countdown uses the date where your browser is running. If you and a friend are in different time zones, you can see different numbers at the same moment without either countdown being wrong.
Compare dates before comparing numbers
Suppose the birthday is January 2. On a device whose local date is December 31, two calendar days remain. Once that device reaches January 1, one remains. A friend whose device has already reached January 2 sees zero. These are whole-date counts, so the result says nothing about the number of hours until a party.
Enter month 1 and day 2 in Birthday Countdown. Calculate As Of initially uses the browser's local date. To reproduce the two-day example, set it to December 31 of the preceding year. The tool has no destination time-zone selector. You can also check the interval using Milestone Date by adding two days to December 31.
Plan for the recipient
For a call across time zones, use the recipient's local birthday in a calendar that supports timed appointments and time zones. This site helps find the date and day count. A saved all-day birthday does not express a particular hour.
Reload the calculator after midnight if you need a fresh result. A number left on screen is a result from the moment you calculated it. Check the device clock if the displayed date appears unexpected.