What will happen if you are flying east across the International Date Line?

The International Date Line (IDL) is an imaginary line that roughly follows the 180° meridian and separates two consecutive calendar days. Time zones in the Eastern Hemisphere (west of the IDL) are ahead of Coordinated Universal Time (UTC), while those in the Western Hemisphere (east of the IDL) are behind UTC.

When you fly eastward across the IDL (e.g., traveling from Japan to Hawaii), you move from a region where time is ahead to a region where time is behind. To sync with local time, you must adjust your calendar backward by one day (such as jumping from Tuesday back to Monday). By subtracting a day from your calendar, you effectively lose 24 hours. Conversely, traveling westward across the IDL requires skipping ahead by adding a day, meaning your calendar gains 24 hours.