Implement missing DateTimeOffset functionality for the Python target

[ncave]

[Python] Implement missing DateTimeOffset members

Summary

The Python target's DateTimeOffset implementation in fable_library/date_offset.py is incomplete. Many members from System.DateTimeOffset either throw at runtime (referenced in Replacements.fs but not implemented in the library) or are silently missing.

Currently working

• Constructor: DateTimeOffset(year, month, day, hour, minute, second, millisecond, offset) via create
• Properties: Year, Month, Day, Hour, Minute, Second, Offset, DateTime, Now, UtcNow
• Static fields: MinValue
• Methods: ToString, TryParse, FromUnixTimeMilliseconds, FromUnixTimeSeconds, ToUnixTimeMilliseconds, ToUnixTimeSeconds
• Operator: op_subtraction (DateTimeOffset - DateTimeOffset → TimeSpan, DateTimeOffset - TimeSpan → DateTimeOffset)
• Equality/comparison: delegated to date.equals / date.compare

Missing / broken

Constructors

• DateTimeOffset(DateTime) — fromDate referenced in Replacements.fs but missing in date_offset.py
• DateTimeOffset(Int64 ticks, TimeSpan offset) — fromTicks referenced but missing
• DateTimeOffset(DateOnly, TimeOnly, TimeSpan) — not handled

Fields

• MaxValue — date_offset.maxValue referenced in Replacements.fs but missing in date_offset.py
• UnixEpoch

Properties

• UtcDateTime — to_universal_time referenced but missing
• LocalDateTime — to_local_time referenced but missing
• UtcTicks — getUtcTicks referenced but missing
• Date (date-only component as DateTime)
• DayOfWeek
• DayOfYear
• Millisecond
• Microsecond
• Nanosecond
• Ticks (clock time ticks)
• TimeOfDay (as TimeSpan)
• TotalOffsetMinutes

Methods

• Add(TimeSpan) — would go via catch-all to date_offset.add, which is missing
• AddDays(float) — catch-all, not implemented
• AddHours(float) — catch-all, not implemented
• AddMicroseconds(float) — catch-all, not implemented
• AddMilliseconds(float) — referenced as addMilliseconds (used for AddTicks), missing
• AddMinutes(float) — catch-all, not implemented
• AddMonths(int) — catch-all, not implemented
• AddSeconds(float) — catch-all, not implemented
• AddYears(int) — catch-all, not implemented
• AddTicks(int64) — dispatches to addMilliseconds, which is missing
• Compare(DateTimeOffset, DateTimeOffset) — static method, not handled
• EqualsExact(DateTimeOffset) — not handled
• FromFileTime(int64) — not handled
• ToFileTime() — not handled
• ToLocalTime() — to_local_time referenced but missing
• ToOffset(TimeSpan) — not handled
• ToUniversalTime() — to_universal_time referenced but missing
• ParseExact — not handled
• TryParseExact — not handled

Operators

• op_addition (DateTimeOffset + TimeSpan) — commented out in Rust extension, missing in date_offset.py

Other

• GetHashCode / hash support — Replacements.fs routes to date.dateHash, which does not exist in date.py; DateTimeOffset values cannot be used as map/set keys reliably

Implementation notes

• The catch-all branch in Replacements.fs (line ~3022) forwards unknown instance methods to date_offset.<lowerFirst(methodName)>, so many Add* methods will silently try to call missing functions at runtime.
• to_local_time and to_universal_time should return DateTimeOffset (not DateTime), consistent with the .NET API.
• AddTicks is currently mapped to addMilliseconds(ticks / 10000) — this function needs to be added, and the mapping should be verified for precision.
• Equality semantics: .NET compares DateTimeOffset by UTC-normalized instant (not by local time + offset pair). The current delegation to date.equals needs to apply the offset before comparing.
• The Rust extension (src/fable-library-py/src/datetime_offset.rs) has stubs for most functions commented out — these should be completed when the Python implementation is stabilized.

Files to change

• src/fable-library-py/fable_library/date_offset.py — main implementation
• src/Fable.Transforms/Python/Replacements.fs — wire up new functions
• tests/Python/TestDateTimeOffset.fs — add test coverage for each implemented member

[ncave]

/repo-assist

[dbrattli]

Added a PR #4477. The 3 remaining are:

• Nanosecond — Python datetime has microsecond precision max
• FromFileTime/ToFileTime — Windows-specific APIs, not in JS target either
• ParseExact/TryParseExact — complex format parsing, not in JS target either