This is a test-only change.

CultureInfo has this documentation: The user might choose to override some of the values associated with the current culture of Windows through the regional and language options portion of Control Panel. For example, the user might choose to display the date in a different format or to use a currency other than the default for the culture. If the culture identifier associated with name matches the culture identifier of the current Windows culture, this constructor creates a CultureInfo object that uses those overrides, including user settings for the properties of the DateTimeFormatInfo instance returned by the DateTimeFormat property, and the properties of the NumberFormatInfo instance returned by the NumberFormat property.

This was causing one test on my machine to fail:

Microsoft.CodeAnalysis.CSharp.ExpressionEvaluator.UnitTests.ExpansionTests.Primitives

Assert.Equal() Failure\r\n           ↓ (pos 1)\r\nExpected: {1/1/0001 12:00:00 AM}\r\nActual:   {0001-01-01 00:00:00}\r\n           ↑ (pos 1)

   at Microsoft.CodeAnalysis.ExpressionEvaluator.ResultProviderTestBase.Verify(DkmEvaluationResult actual, DkmEvaluationResult expected)
   at Microsoft.CodeAnalysis.CSharp.ExpressionEvaluator.UnitTests.ExpansionTests.Primitives() in C:\c\roslyn\src\ExpressionEvaluator\CSharp\Test\ResultProvider\ExpansionTests.cs:line 62

The fix is to explicitly override this behavior, by passing useUserOverride = false in a different constructor.

Behavior testing in a console app:

Console.WriteLine(new DateTime());
CultureInfo.CurrentCulture = new CultureInfo("en-US");
Console.WriteLine(new DateTime());
CultureInfo.CurrentCulture = new CultureInfo("en-US", false);
Console.WriteLine(new DateTime());

produces

0001-01-01 00:00:00
0001-01-01 00:00:00
1/1/0001 12:00:00 AM

I don't know what team should review this, so I've just added roslyn-compiler. Feel free to add a area-* and the correct reviewer if there is a more appropriate team.