Add In() method to Date type for converting between IANA timezones.
Implements comprehensive DST handling with proper wall-clock time
preservation across daylight saving transitions.
Features:
- In(location) converts to specified IANA timezone
- Returns ErrInvalidTimezone for invalid timezone names
- Never panics on invalid input
- Preserves language settings across conversions
- Maintains immutability pattern
DST Handling:
- Add(1, Days) preserves wall clock time, not duration
- Tested across spring forward (Mar 29, 2026)
- Tested across fall back (Oct 25, 2026)
Testing:
- 100% coverage for In() method
- 6 comprehensive test functions (228 lines)
- Tests for Europe/Berlin, America/New_York, Asia/Tokyo, UTC
- Error handling tests (empty string, invalid timezones)
- Immutability and language preservation tests
- 3 example tests demonstrating usage
Overall coverage: 98.1%
- Add Date struct with time.Time and Lang fields
- Implement package-level constructors: Now(), From(time.Time)
- Add conversion methods: Time(), Unix(), FromUnix(int64)
- Support negative Unix timestamps (dates before 1970)
- Support full time.Time range (year 0001-9999+)
- Add WithLang() for i18n support (placeholder)
- Create package documentation in quando.go
- Comprehensive unit tests with 100% coverage
- Example tests for godoc
- Verify immutability through tests
All acceptance criteria met:
✓ Date struct defined with time.Time and Lang fields
✓ Now() returns current date
✓ From(time.Time) converts to Date
✓ Time() extracts underlying time.Time
✓ Unix() returns Unix timestamp (int64)
✓ FromUnix(int64) creates Date from timestamp
✓ Unit tests with 100% coverage (exceeds 95% requirement)
✓ Godoc comments for all exported types/functions
✓ Example tests in example_test.go
