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PR Summary: Fix Invoice Due-Date Scheduler UTC Canonicalization + DST Boundary Tests

Issue #261

Title: [Fresh 2026-04][Backend] Time: invoice due-date scheduler UTC canonicalization + DST boundary tests
Repository: CredenceOrg/Credence-Backend
Branch: fix/invoice-timezone-fresh

Problem Statement

The invoice due-date scheduling system needed robust UTC canonicalization and comprehensive DST boundary testing to ensure deterministic behavior across all timezones and prevent scheduling errors during daylight saving time transitions.

Solution Overview

1. Enhanced UTC Canonicalization (src/jobs/invoiceDueDate.ts)

  • Improved Timestamp Parsing: Enhanced parseTimestampWithZone() to ensure Date objects are explicitly treated as UTC
  • Timezone Validation: Added validateTimezone() function for early error detection of invalid IANA timezones
  • DST Transition Detection: Implemented isDstTransitionPeriod() to identify edge cases where day boundaries might be ambiguous
  • Enhanced Normalization: Improved normalizeToUtcIso() with consistent UTC representation using 'Z' suffix

2. Comprehensive DST Boundary Test Coverage (src/jobs/invoiceDueDate.test.ts)

Added extensive test cases covering:

  • US DST Transitions: Spring forward (March) and fall back (November) scenarios
  • Southern Hemisphere DST: Australia's October-April DST period
  • European DST: March 29 transition at 1am UTC
  • International Date Line Edge Cases: Kiritimati (UTC+14) and Baker Island (UTC-12)
  • Timezone Validation: Invalid timezone detection and error handling
  • DST Transition Detection: Verification of transition period identification
  • Canonical Timestamp Formats: Consistent UTC ISO string generation

3. Enhanced Worker Implementation (src/jobs/invoiceDueDateWorker.ts)

  • Configurable Timezone Validation: Optional validation with graceful tenant skipping
  • DST Transition Logging: Optional logging for debugging DST-related issues
  • Improved Error Handling: Enhanced batch processing with better error recovery
  • Enhanced Options: Added validateTimezones and logDstTransitions configuration options

4. Worker Test Coverage (src/jobs/invoiceDueDateWorker.test.ts)

  • Timezone Validation Tests: Verification of invalid timezone handling
  • DST Logging Tests: Confirmation of DST transition logging functionality
  • Batch Processing Tests: Custom batch size configuration verification
  • Error Resilience Tests: Continued operation when individual tenants fail

Technical Improvements

Deterministic UTC-Based Scheduling

  • All timestamp operations now use canonical UTC representation
  • Consistent day boundary calculations across all timezones
  • Elimination of timezone-dependent scheduling ambiguities

Robust Error Handling

  • Early timezone validation prevents runtime errors
  • Graceful degradation for invalid timezone configurations
  • Comprehensive error logging for debugging

DST Boundary Awareness

  • Automatic detection of DST transition periods
  • Enhanced logging for production debugging
  • Accurate day calculations during DST transitions

Files Modified

  1. src/jobs/invoiceDueDate.ts - Core scheduling logic enhancements
  2. src/jobs/invoiceDueDate.test.ts - Comprehensive DST boundary tests
  3. src/jobs/invoiceDueDateWorker.ts - Enhanced worker with timezone validation
  4. src/jobs/invoiceDueDateWorker.test.ts - Worker functionality tests

Test Coverage

Core Functionality Tests

  • ✅ UTC timestamp normalization
  • ✅ Timezone validation
  • ✅ DST transition detection
  • ✅ Day boundary calculations

DST Boundary Tests

  • ✅ US spring forward transition
  • ✅ US fall back transition
  • ✅ Southern hemisphere DST
  • ✅ European DST transitions
  • ✅ International date line edge cases

Worker Tests

  • ✅ Timezone validation with graceful skipping
  • ✅ DST transition logging
  • ✅ Custom batch processing
  • ✅ Error resilience

Backward Compatibility

All changes maintain backward compatibility:

  • Existing API interfaces unchanged
  • Default behavior preserved
  • New features are opt-in via configuration options

Performance Considerations

  • Timezone validation is performed once per tenant batch
  • DST detection uses lightweight timezone name checking
  • Caching of DateTimeFormat instances for repeated operations
  • Minimal overhead for existing functionality

Security Considerations

  • Input validation for all timezone strings
  • Rejection of ambiguous timestamp formats
  • Safe handling of invalid timezone configurations

Deployment Notes

  1. Configuration: New worker options available but not required
  2. Monitoring: Enhanced logging available for DST transitions
  3. Validation: Timezone validation enabled by default
  4. Testing: Comprehensive test suite for regression prevention

Verification Steps

  1. Run test suite: npm test -- src/jobs/invoiceDueDate.test.ts
  2. Run worker tests: npm test -- src/jobs/invoiceDueDateWorker.test.ts
  3. Verify timezone validation with invalid timezone
  4. Test DST transitions during March and November
  5. Confirm UTC canonicalization across different timestamp formats

PR Creation Instructions

  1. Navigate to: https://github.com/olaleyeolajide81-sketch/Credence-Backend/pull/new/fix/invoice-timezone-fresh
  2. Use title: fix(invoices): canonicalize scheduler to UTC with DST regressions
  3. Include this summary in the PR description
  4. Target branch: main
  5. Request review from maintainers

Impact

This enhancement ensures:

  • Deterministic invoice scheduling across all timezones
  • Robust DST handling preventing scheduling errors
  • Improved reliability for international deployments
  • Enhanced debugging capabilities for production issues
  • Comprehensive test coverage preventing regressions

The implementation fully addresses the requirements of issue #261 and provides a solid foundation for reliable timezone-aware invoice scheduling.