The Earth rotates once about every 24 hours. If everyone used the same clock time, noon would occur at night for some regions. Time zones solve this by letting different longitudes set their clocks to match daylight and daily life.
In theory, each time zone would be 15 degrees of longitude wide (24 zones × 15° = 360°). In reality, borders, geography, and politics shape time zone lines, so they often zig-zag.
Some regions use offsets like UTC+5:30 or UTC+9:30 to better align with their location. This is normal and important to account for when calculating time differences.
The International Date Line is the boundary where the calendar day changes. Crossing it east or west can add or subtract a day. This matters for flights and planning international events.
Always specify the time zone. “10:00 AM” alone is ambiguous; “10:00 AM Tokyo time (UTC+9)” is not.