fix(schema): reject year 0000 in dateTime, date, gYearMonth, gYear

XSD 1.0 has no year zero (the year before 0001 is -0001), so 0000-01-01T00:00:00
and the other 0000 forms are not valid values. DateTimeParser.parseYearOnly now
rejects a zero magnitude for both signs, so negative years like -0001 still parse.

Closes the XSTS dateTime011 false negative: invalid-instances-accepted 31 -> 30,
no other bucket moved (valid-schemas-rejected and valid-instances-rejected held at
0). 1449 unit tests pass; the full XSTS suite settles at 0/43/0/30.

CHANGELOG.md

@@ -28,6 +28,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- `dateTime`, `date`, `gYearMonth`, and `gYear` now reject the year `0000`. XSD
+  1.0 has no year zero (the year before `0001` is `-0001`), so `0000-01-01T00:00:00`
+  and the other `0000` forms are not valid values. `DateTimeParser.parseYearOnly`
+  rejects a zero magnitude for both signs, so `-0001` and other negative years
+  still parse. XSTS invalid-instances-accepted 31 -> 30 (`dateTime011`), no other
+  bucket moved (valid-rejected held at 0).
+
 - Fixed a dead `rootLocation` fallback in `XSDParser.containerLocationMap`. The
   map value type is itself `String?`, so `loc ?? rootLocation` bound the
   coalescing at the outer optional and never used `rootLocation` (the compiler

Sources/Schema/DateTimeParser.swift

@@ -54,7 +54,10 @@ extension PureXML.Schema {
         private mutating func parseYearOnly() -> DateTimeValue? {
             var value = DateTimeValue()
             let negative = consume("-")
-            guard let digits = readDigits(minimum: 4) else { return nil }
+            // XSD 1.0 has no year zero: the lexical year 0000 (and -0000) is not a
+            // valid value for dateTime, date, gYearMonth, or gYear. `digits` is the
+            // magnitude, so rejecting 0 covers both signs.
+            guard let digits = readDigits(minimum: 4), digits != 0 else { return nil }
             value.year = (negative ? -1 : 1) * digits
             return value
         }

Tests/XSDDatatypeTests.swift

@@ -57,6 +57,18 @@ struct XSDDatatypeTests {
         #expect(!valid("2026-06-08T12:30:00+15:00", .dateTime))
     }
 
+    @Test("XSD 1.0 has no year zero: 0000 is rejected across the year-bearing types")
+    func test_yearZero() {
+        // Rejected: the prohibited year 0000 (and -0000) wherever a year appears.
+        #expect(!valid("0000-01-01T00:00:00", .dateTime))
+        #expect(!valid("0000-01-01", .date))
+        #expect(!valid("0000-01", .gYearMonth))
+        #expect(!valid("0000", .gYear) && !valid("-0000", .gYear))
+        // Accepted: the smallest magnitudes either side of zero, and a normal year.
+        #expect(valid("0001-01-01T00:00:00", .dateTime) && valid("-0001-12-31", .date))
+        #expect(valid("0001", .gYear) && valid("2026", .gYear) && valid("-0044-03", .gYearMonth))
+    }
+
     @Test("the g* date types parse their reduced forms")
     func test_gTypes() {
         #expect(valid("--02-29", .gMonthDay) && !valid("--02-30", .gMonthDay))

Tests/XSTSSuiteTests.swift

@@ -150,13 +150,13 @@ struct XSTSSuiteTests {
     private let knownSchemaValidRejected = 0
     private let knownSchemaInvalidAccepted = 43
     private let knownInstanceValidRejected = 0
-    /// Instance bucket (133 -> 31), per-step deltas in CHANGELOG.md: xsi:type must derive from the
+    /// Instance bucket (133 -> 30), per-step deltas in CHANGELOG.md: xsi:type must derive from the
     /// declared type (complex, then atomic and list/union, resolved through the element-ref chain);
     /// block/blockDefault (incl. "substitution" and an untyped head) bar substitutions after the
     /// derivation backbone was re-keyed by namespace; ur-type `xsd:anyType` wildcards are `lax`;
     /// identity fields compare in the decimal value space and take a declared default/fixed when
-    /// absent or empty; a complex type's attribute wildcard is the INTERSECTION of its sources.
-    private let knownInstanceInvalidAccepted = 31
+    /// absent/empty; an attribute wildcard is the INTERSECTION of its sources; year 0000 is invalid.
+    private let knownInstanceInvalidAccepted = 30
 
     @Test("Every XSTS case behaves: compile, reject, validate, invalidate")
     func test_suite() throws {

docs/plans/2026-06-18-false-negative-tail.md

@@ -94,7 +94,7 @@ classification is wrong.
 | Nillability | `nillable00201m/nillable00201m2_n` | `XSDParserHelpers`, instance validator |
 | Type derivation | `typedef00204m/typeDef00204m1_n` | `ComplexValidatorXSIType`, derivation tables |
 | Additional-suite instance triage | `addB065/addB065.i` | to classify from fixture |
-| Datatype value-space edges | `anyURI_a004_1339/anyURI_a004_1339.i`, `dateTime011_2008/dateTime011_2008.i` | `SimpleType` |
+| Datatype value-space edges | `anyURI_a004_1339/anyURI_a004_1339.i` (open); ~~`dateTime011_2008/dateTime011_2008.i`~~ closed: XSD 1.0 has no year zero, `0000` now rejected | `SimpleType`, `DateTimeParser` |
 | Element constraints | `elemO011/elemO011.i`, `elemT074/elemT074.i` | instance validator, element declarations |
 | Identity constraints | `idG006/idG006.i`, `idK012/idK012.i`, `idZ010/idZ010.i`, `idZ012/idZ012.i` | `IdentityValidator*` |
 | Model group instance matching | `mgO013/mgO013.i`, `mgZ001/mgZ001.i` | `ContentMatcher`, `ParticleRestriction` fallout |