- Add Clock interface with Now() and From(t time.Time) methods
- Implement DefaultClock using time.Now() for production code
- Implement FixedClock with fixed time for deterministic testing
- Add factory functions NewClock() and NewFixedClock(time.Time)
- Comprehensive unit tests demonstrating deterministic test patterns
- Edge case testing (epoch, year 0001, year 9999, nanoseconds)
- Timezone preservation tests
- Example tests showing test usage patterns
- Performance benchmarks for both clock implementations
- 100% test coverage (exceeds 95% requirement)
All acceptance criteria met:
✓ Clock interface defined
✓ DefaultClock implementation using time.Now()
✓ FixedClock implementation with fixed time
✓ NewClock() factory function
✓ NewFixedClock(time.Time) factory function
✓ Unit tests demonstrating deterministic test patterns
✓ Godoc comments
✓ Example test showing test usage pattern
