Format UTC offsets with the invariant culture

docs/type-to-string-mapping.md

@@ -267,6 +267,10 @@ public static partial class DateFormatter
         return value.ToString("yyyy-MM-ddTHH-mm-ss.FFFFFFF", Culture.InvariantCulture);
     }
 
+    // Interpolation formats with the current culture, and NumberFormatInfo.NegativeSign is not "-"
+    // everywhere: sv-SE renders U+2212 and ar-SA prefixes U+061C. A negative offset carries that
+    // sign into both snapshot content and parameter file names, so the culture is pinned here the
+    // same way it is for every date part above
     static string GetDateOffset(DateTimeOffset value)
     {
         var offset = value.Offset;
@@ -275,27 +279,28 @@ public static partial class DateFormatter
         {
             if (offset.Minutes == 0)
             {
-                return $"+{offset.TotalHours:0}";
+                return FormattableString.Invariant($"+{offset.TotalHours:0}");
             }
 
-            return $"+{offset.Hours:0}-{offset.Minutes:00}";
+            return FormattableString.Invariant($"+{offset.Hours:0}-{offset.Minutes:00}");
         }
 
         if (offset < TimeSpan.Zero)
         {
             if (offset.Minutes == 0)
             {
-                return $"{offset.Hours:0}";
+                return FormattableString.Invariant($"{offset.Hours:0}");
             }
 
-            return $"{offset.Hours:0}{offset.Minutes:00}";
+            // Minutes is negative too, which is what renders the separator
+            return FormattableString.Invariant($"{offset.Hours:0}{offset.Minutes:00}");
         }
 
         return "+0";
     }
 }
 ```
-<sup><a href='/src/Verify/Serialization/DateFormatter_DateTimeOffset.cs#L1-L86' title='Snippet source file'>snippet source</a> | <a href='#snippet-DateFormatter_DateTimeOffset.cs' title='Start of snippet'>anchor</a></sup>
+<sup><a href='/src/Verify/Serialization/DateFormatter_DateTimeOffset.cs#L1-L91' title='Snippet source file'>snippet source</a> | <a href='#snippet-DateFormatter_DateTimeOffset.cs' title='Start of snippet'>anchor</a></sup>
 <!-- endSnippet -->
 
 

src/Verify.Tests/DateFormatterTests.cs

@@ -28,6 +28,39 @@ public Task DateTimeOtherTimeZoneNegativeToParameterString()
         return Verify(DateFormatter.ToParameterString(date));
     }
 
+    // NumberFormatInfo.NegativeSign is not "-" in every culture: sv-SE renders U+2212 and ar-SA
+    // prefixes U+061C to it. The offset is both snapshot content and part of a parameter file
+    // name, so a negative offset rendered on one machine has to equal the snapshot committed
+    // from another. The sign is set explicitly rather than by picking a real culture, so the
+    // assertion does not move with the ICU data the test happens to run against
+    [Theory]
+    [InlineData(-1.5, "2000-10-01 -1-30", "2000-10-01-1-30")]
+    [InlineData(-5, "2000-10-01 -5", "2000-10-01-5")]
+    [InlineData(1.5, "2000-10-01 +1-30", "2000-10-01+1-30")]
+    [InlineData(5, "2000-10-01 +5", "2000-10-01+5")]
+    [InlineData(0, "2000-10-01 +0", "2000-10-01+0")]
+    public void OffsetDoesNotTakeTheCurrentCultureNegativeSign(double hours, string expectedJson, string expectedParameter)
+    {
+        var date = new DateTimeOffset(2000, 10, 1, 0, 0, 0, TimeSpan.FromHours(hours));
+
+        // U+2212 MINUS SIGN, as sv-SE uses
+        var culture = (CultureInfo) CultureInfo.InvariantCulture.Clone();
+        culture.NumberFormat.NegativeSign = "−";
+
+        var original = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo.CurrentCulture = culture;
+
+            Assert.Equal(expectedJson, DateFormatter.Convert(date));
+            Assert.Equal(expectedParameter, DateFormatter.ToParameterString(date));
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = original;
+        }
+    }
+
     [Fact]
     public Task DateTimeLocalToJsonString()
     {

src/Verify/Serialization/DateFormatter_DateTimeOffset.cs

@@ -57,6 +57,10 @@ static string GetParameterDatePart(DateTimeOffset value)
         return value.ToString("yyyy-MM-ddTHH-mm-ss.FFFFFFF", Culture.InvariantCulture);
     }
 
+    // Interpolation formats with the current culture, and NumberFormatInfo.NegativeSign is not "-"
+    // everywhere: sv-SE renders U+2212 and ar-SA prefixes U+061C. A negative offset carries that
+    // sign into both snapshot content and parameter file names, so the culture is pinned here the
+    // same way it is for every date part above
     static string GetDateOffset(DateTimeOffset value)
     {
         var offset = value.Offset;
@@ -65,20 +69,21 @@ static string GetDateOffset(DateTimeOffset value)
         {
             if (offset.Minutes == 0)
             {
-                return $"+{offset.TotalHours:0}";
+                return FormattableString.Invariant($"+{offset.TotalHours:0}");
             }
 
-            return $"+{offset.Hours:0}-{offset.Minutes:00}";
+            return FormattableString.Invariant($"+{offset.Hours:0}-{offset.Minutes:00}");
         }
 
         if (offset < TimeSpan.Zero)
         {
             if (offset.Minutes == 0)
             {
-                return $"{offset.Hours:0}";
+                return FormattableString.Invariant($"{offset.Hours:0}");
             }
 
-            return $"{offset.Hours:0}{offset.Minutes:00}";
+            // Minutes is negative too, which is what renders the separator
+            return FormattableString.Invariant($"{offset.Hours:0}{offset.Minutes:00}");
         }
 
         return "+0";

src/todo.md

@@ -29,8 +29,8 @@ All six resolved 2026-08-16 (five fixed here; the inline item resolved as by-des
 
 ## Correctness, narrower triggers
 
-- [ ] **Negative UTC offsets formatted with current culture.**
-  `Verify/Serialization/DateFormatter_DateTimeOffset.cs:78,81` — plain interpolation (`$"{offset.Hours:0}"`) uses `CurrentCulture` while every other call in the file passes `Culture.InvariantCulture`. Under `ar-SA` the negative sign renders as invisible U+061C + `-`, so a `DateTimeOffset` parameter with offset `-05:00` produces a filename that never matches a snapshot committed from an en-US machine. Also leaks into snapshot content via `Convert` when date scrubbing is off.
+- [x] **Negative UTC offsets formatted with current culture.**
+  `Verify/Serialization/DateFormatter_DateTimeOffset.cs:78,81` — plain interpolation (`$"{offset.Hours:0}"`) uses `CurrentCulture` while every other call in the file passes `Culture.InvariantCulture`. Under `ar-SA` the negative sign renders as invisible U+061C + `-`, so a `DateTimeOffset` parameter with offset `-05:00` produces a filename that never matches a snapshot committed from an en-US machine. Also leaks into snapshot content via `Convert` when date scrubbing is off. Wider than first recorded: `sv-SE` renders U+2212 MINUS SIGN and `fa-IR` U+200E + U+2212, so the sign is visibly wrong, not just invisible.
 
 - [ ] **Sub-millisecond date parameters collide.**
   `Verify/Serialization/DateFormatter_DateTime.cs` (and the `DateTimeOffset` twin) use `Second == 0` / `Millisecond == 0` to omit the fraction, but sub-millisecond ticks leave those properties 0. `AddTicks(1)` and `AddTicks(2)` cases format identically → spurious "prefix has already been used" (or silent sharing of one verified file). Correct check is ticks-based.