Timekeeping & Dateline – International Date Line (IDL) What is the time difference when you cross from one side of the International Date Line to the other (east–west or west–east), considering the calendar day shift?

Introduction / Context:
The International Date Line (IDL), roughly along 180° longitude with deviations for political boundaries, separates calendar days. Understanding the time and date change at this line is fundamental in world timekeeping and navigation.

Given Data / Assumptions:

• Crossing the IDL involves a calendar adjustment: one day is gained or lost.
• Standard time zones are based on 15° increments (≈1 hour), but the IDL is about the calendar day, not just clock hours.
• We need the net difference between calendar days across the line.

Concept / Approach:
While time zones step hour by hour, the IDL marks the boundary where the date jumps by 24 hours. Crossing eastward means subtracting a day; crossing westward means adding a day, keeping local clock times aligned with neighboring zones but adjusting the calendar.

Step-by-Step Solution:

Verification / Alternative check:
Airline itineraries crossing the Pacific often show arriving “earlier” or “later” by a day; navigation manuals formalize the 24-hour date change at the IDL.

Why Other Options Are Wrong:

• 12/36/48 hours: Do not correspond to the single-day (24 h) calendar step at the IDL.

Common Pitfalls:
Confusing clock time differences with the date shift. The time of day may be similar, but the calendar is different by one whole day across the line.

Final Answer:
24 hours