What defines a day - When technical implementation affects business behaviour

Short summary

A calendar bug where dates shift across timezones reveals a core design lesson: code must reflect real-world user expectations, not implementation convenience. The bug occurred because JavaScript Date objects represent moments in time, not calendar days—when saved using UTC methods from local times, dates shifted by timezone offset. The fix requires consistency (always local or always UTC) and aligning technical decisions with business intent, a principle from Domain-Driven Design.

• •JavaScript Date objects represent moments in time, not calendar days—serializing local times as UTC shifts them by timezone offset
• •Cross-team misalignment caused the bug: UI treated dates locally, persistence used UTC; consistency is the fix
• •Design code to match business intent and user expectations, not implementation convenience—a Domain-Driven Design principle

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