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2026-04-12 | 🕐 Working Entirely in Pacific Time 🤖

🎯 The Problem

🔄 The blog series config files each carried a field called postTimeUtc alongside scheduleHourPacific. 📊 For example, a series scheduled at 6 AM Pacific had postTimeUtc set to 14 colon 00, which is 6 plus 8. 🤔 The problem is that this addition only holds during Pacific Standard Time. 🌞 During Pacific Daylight Time the offset shrinks from 8 hours to 7 hours, making the hardcoded UTC value wrong for roughly half the year.

🐛 This meant that comment filtering, which uses the time to build a cutoff timestamp, was off by an hour during daylight saving time. 📝 Comments posted between the real publication time and the stale cutoff would be incorrectly excluded from the AI prompt context.

💡 The Solution

🧭 Instead of converting Pacific times to UTC and comparing UTC strings, we now do all datetime work in Pacific time. 📦 The tz library provides the IANA timezone database and functions like utcToLocalTimeTZ that convert external UTC timestamps into Pacific local times. 🏠 This means our program never needs to think in UTC at all.

🔧 When comment timestamps arrive from GitHub in UTC format, we parse them into UTCTime values and immediately convert to Pacific LocalTime using toPacificLocalTime. 📅 The schedule cutoff is simply a LocalTime built from the post date and the schedule TimeOfDay. 🎯 Comparison happens entirely in Pacific time, which is the timezone all our business logic operates in.

🧹 What Changed

📦 Five JSON config files in the series directory lost their postTimeUtc field. 🗑️ The RawConfig parser no longer expects or parses it. 🗑️ The DiscoveredSeries record no longer carries it. 🔄 BlogSeriesConfig replaced its bscPostTimeUtc text field with bscScheduleTime as a TimeOfDay, which is the proper domain type for representing a time of day.

🔧 The filterCommentsAfterLastPost function in BlogPrompt now works entirely in Pacific time. 📅 It constructs a Pacific LocalTime cutoff from the post date and schedule time, parses each comment’s UTC timestamp using iso8601ParseM, converts it to Pacific time using toPacificLocalTime, and compares the two LocalTime values directly. 🗑️ The old pacificToUtcHour function and the formatHourMinute helper were both removed since we no longer need to convert anything to UTC.

🏗️ The PacificTime module was further simplified. 🆕 It now exports toPacificLocalTime, a one-line wrapper around utcToLocalTimeTZ with the Pacific timezone. 🗑️ The pacificToUtcHour function and the localTimeToUTCTZ import were both removed. 📏 The module is now 42 lines, down from 82 before the tz library migration.

🧪 Testing

✅ Nine new tests were added for toPacificLocalTime in PacificTimeTest, covering both PST and PDT dates, day boundary crossings, and property-based tests. 🔬 One property verifies the result day is always within 1 day of the UTC day. 📐 Two other properties verify the specific offsets: PST dates subtract 8 hours, PDT dates subtract 7 hours.

📊 All 1570 tests pass. 🧹 Zero hlint hints.

🔬 Design Decisions

🧭 The guiding principle is that all datetime work happens in Pacific time. 📥 External UTC timestamps are converted to Pacific at the boundary, and from that point on everything is compared in Pacific. 🧅 This follows the functional core, imperative shell principle: the pure comparison logic never touches UTC.

🏷️ Replacing the Int hour with TimeOfDay follows the domain types over primitives rule. 📦 TimeOfDay is already used elsewhere in the codebase for similar purposes, such as the posting cutoff in ContentDiscovery. 🔧 The JSON config still stores an integer for the hour, but the validation layer immediately constructs a TimeOfDay from it.

🛡️ Comments whose timestamps cannot be parsed are included rather than excluded. 📅 In practice this should never happen since GitHub always produces valid ISO 8601 timestamps, but the safe default prevents silent data loss.

📚 Book Recommendations

📖 Similar

• Haskell Programming from First Principles by Christopher Allen and Julie Moronuki is relevant because it covers pure functions, algebraic data types, and the type-driven development approach used throughout this change.
• Real World Haskell by Bryan O’Sullivan, Don Stewart, and John Goerzen is relevant because it demonstrates practical Haskell patterns for data transformation and time handling.

↔️ Contrasting

• JavaScript: The Good Parts by Douglas Crockford offers a contrasting view where timezone handling is typically done through mutable Date objects and library wrappers rather than pure algebraic computation.

🔗 Related

• Why Time Flies by Alan Burdick explores human perception of time and the surprisingly complex nature of measuring and converting between time systems.
• Domain-Driven Design by Eric Evans is relevant because replacing a redundant configuration field with a derived computation is a classic example of making implicit domain knowledge explicit.