Add DateTimeOffset raw-value support to ${date} / ${longdate} (or add a new ${datetimeoffset} renderer) #6161

Background

Issue #6115 / PR #6116 added typed DateTimeOffset flow through DatabaseTarget when DatabaseParameterInfo.ParameterType = typeof(DateTimeOffset) and the layout's raw value is already a DateTimeOffset object.

That fix works great when the user populates LogEventInfo.Properties["Timestamp"] = new DateTimeOffset(e.TimeStamp) and uses ${event-properties:item=Timestamp} as the layout — the typed object flows through to the SqlParameter without a string round-trip.

Problem

LogEventInfo.TimeStamp is a DateTime, and ${date} / ${longdate} only expose it as a DateTime raw value. To target a SQL DATETIMEOFFSET column from e.TimeStamp, callers must:

1. Manually populate an event property in every log call (or via a wrapper):

   e.Properties["Timestamp"] = new DateTimeOffset(e.TimeStamp);

2. Reference it via ${event-properties:item=Timestamp} instead of the built-in ${date}.

This is boilerplate that exists solely to bridge DateTime → DateTimeOffset for the typed parameter pipeline. Using ${date:format=o} defeats the raw-value path (forces string rendering), and relying on ParameterType = typeof(DateTimeOffset) to convert from a DateTime raw value loses fidelity for users whose TimeSource is configured for a specific kind.

Proposed Enhancement

Either of the following would eliminate the workaround:

Option A (preferred — minimal surface area)

Add an asDateTimeOffset (bool) property to DateLayoutRenderer (and LongDateLayoutRenderer). When true, the renderer's IRawValue.TryGetRawValue returns a DateTimeOffset constructed from LogEventInfo.TimeStamp (using TimeZoneInfo.Local for Kind.Local / Kind.Unspecified and TimeSpan.Zero for Kind.Utc). String rendering remains unchanged.

<parameter name="@Timestamp"
           layout="${date:asDateTimeOffset=true}"
           parameterType="System.DateTimeOffset" />

Option B (purely additive)

Add a dedicated ${datetimeoffset} layout renderer that always exposes e.TimeStamp as a typed DateTimeOffset raw value, with an optional format for string rendering.

Why this fits NLog's model

• It follows the same pattern PR PropertyTypeConverter - Prioritize IFormattable ToString when Format specified #6116 established: typed values flow end-to-end without string round-trips.
• It uses the existing IRawValue plumbing — no new abstraction needed.
• It keeps file/console targets working (string rendering is unchanged) while enabling SQL DATETIMEOFFSET columns to receive a native typed value.
• It removes the only remaining reason to push timestamps through Properties, which is a documented pain point for SQL DatabaseTarget users.

Alternatives considered

• Setting e.Properties["Timestamp"] = new DateTimeOffset(e.TimeStamp) — works today but is per-call boilerplate.
• ${date:format=o} + ParameterType=DateTimeOffset — round-trips through string, defeats the benefit of PropertyTypeConverter - Prioritize IFormattable ToString when Format specified #6116.
• Custom LayoutRenderer subclass — every consumer would have to ship one; better suited as a built-in.

Environment

• NLog: 6.1.2
• NLog.Database: 6.0.3
• Target: SQL Server DATETIMEOFFSET column via Microsoft.Data.SqlClient