Fix #1437: ConvertDateTimeOffset falls back to AsDateTime for timezone-less xs:dateTime values (#1617)

xs:dateTime values without a timezone suffix (e.g. '2022-04-28T10:09:17') are parsed by
ParseISO8601FormattedDateTime as DateTimeKind.Unspecified. AsDateTimeOffset rejects these
(returning None) to keep inference strict, causing a runtime exception when the XSD
declares the field as xs:dateTime.

Fix: In TextRuntime.ConvertDateTimeOffset, fall back to AsDateTime when AsDateTimeOffset
returns None. AsDateTime already converts Unspecified to Local, so DateTimeOffset(dt)
uses the local timezone offset — matching standard xs:dateTime semantics for timezone-free
values.

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>

Author: 3 people

src/FSharp.Data.Runtime.Utilities/TextRuntime.fs

@@ -71,8 +71,16 @@ type TextRuntime =
         |> Option.bind (TextConversions.AsDateTime(TextRuntime.GetCulture cultureStr))
 
     static member ConvertDateTimeOffset(cultureStr, text) =
+        let culture = TextRuntime.GetCulture cultureStr
+
         text
-        |> Option.bind (TextConversions.AsDateTimeOffset(TextRuntime.GetCulture cultureStr))
+        |> Option.bind (fun s ->
+            match TextConversions.AsDateTimeOffset culture s with
+            | Some dto -> Some dto
+            | None ->
+                // Fall back for xs:dateTime values without timezone (DateTimeKind.Unspecified).
+                // AsDateTime converts Unspecified to Local, so DateTimeOffset(dt) uses the local offset.
+                TextConversions.AsDateTime culture s |> Option.map DateTimeOffset)
 
     static member ConvertTimeSpan(cultureStr, text) =
         text

tests/FSharp.Data.Tests/XmlProvider.fs

@@ -1213,6 +1213,14 @@ let ``date is formatted properly``() =
     validXml.XElement.Attribute(XName.Get "date").Value
     |> should equal "2018-08-29"
 
+[<Test>]
+let ``xs:dateTime without timezone parses as DateTimeOffset (issue 1437)``() =
+    // xs:dateTime values without timezone offset (DateTimeKind.Unspecified) should
+    // still parse successfully as DateTimeOffset, treated as local time.
+    let xml = """<A int="0" long="0" date="2022-01-01" dateTime="2022-04-28T10:09:17" boolean="false" decimal="0" double="0" />"""
+    let a = SimpleTypes.Parse(xml)
+    a.DateTime.DateTime |> should equal (System.DateTime(2022, 4, 28, 10, 9, 17))
+
 type TimeSpanXML = XmlProvider<"Data/TimeSpans.xml">
 
 [<Test>]