How a Single Integer Silently Broke a Habit App's Internal Clock

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A developer building a 21-day habit tracking app called Lifemaxxing AI discovered that a single integer, _currentDayIndex, had quietly grown from a simple counter into the app's unofficial calendar. The variable became both a derived value and a manually overridable one, creating an undetected conflict where stored data could mean two different things with no way to distinguish them. Two separate functions in the same codebase calculated the current day from different reference points, meaning they only agreed under conditions that rarely occurred in practice. Additionally, the app used Duration.inDays, which counts elapsed 24-hour periods rather than calendar dates, causing a user's first day to stretch up to 25 hours depending on when they completed onboarding. The developer concluded that derived values should never have setters, and that any value which can be both computed and overwritten becomes a cache without an invalidation rule — effectively a delayed bug.

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