- Add Add(value, unit) method for all 8 units
- Add Sub(value, unit) method (wraps Add with negative value)
- Implement month-end overflow logic for Months/Quarters/Years
- When adding months, if target day doesn't exist, snap to last day of month
- Handle all month-end combinations correctly
- Leap year support (Feb 29 edge cases)
- DST-safe calendar day arithmetic for Days unit
- Negative value support (Add(-1) == Sub(1))
- Method chaining support (fluent API)
- Immutability verified (original date never modified)
- Timezone preservation across operations
- Comprehensive table-driven tests for month-end edge cases
- All 30/31 day month combinations tested
- Leap year tests (Feb 29 -> Feb 28)
- Cross-year boundary tests
- Negative value tests
- Method chaining tests
- Performance benchmarks (all < 1µs)
- 98.8% test coverage (exceeds 95% requirement)
- Godoc with month-end overflow examples
Benchmark results:
- AddDays: ~42ns (< 1µs target) ✓
- AddMonths: ~191ns (< 1µs target) ✓
- AddYears: ~202ns (< 1µs target) ✓
- Method chaining: ~263ns for 3 ops ✓
- Zero allocations for all operations
All acceptance criteria met:
✓ Add() implemented for all 8 units
✓ Sub() implemented for all 8 units
✓ Month-end overflow logic correct
✓ Leap year handling (Feb 29 edge cases)
✓ DST handling (calendar days)
✓ Negative values supported
✓ Method chaining works
✓ Unit tests with 98.8% coverage
✓ Table-driven tests for month-end edge cases
✓ Benchmarks meet <1µs target
✓ Godoc comments with month-end examples
