Timezones & DST: The Jetlag Trap

Timezone bugs are among the hardest to reproduce because they only manifest at specific moments — the exact instant clocks spring forward or fall back, or when a user in Tokyo books a meeting that a colleague in New York sees on the wrong day. These bugs lie dormant for months and then explode in production at 2:00 AM on the second Sunday of March.

This module gives you the mental model and practical tools to test timezone-sensitive systems with confidence. You will understand what UTC offsets are, why Daylight Saving Time creates two genuinely ambiguous moments per year, and how the classic three-timezone problem (browser, server, database) produces date-shifting bugs that are invisible in single-timezone test environments. You will learn which timezones to include in your test matrix, how to craft test cases that exercise DST transition windows, and what automation patterns help you verify timezone correctness without waiting for the real transition to happen.

By the end of this module, you will approach every scheduling, booking, and notification feature with a timezone checklist — and you will know exactly what to look for when a reported bug says "the date is wrong."