Fix XSLT format-date and format-time to output only date or time with empty format string

src/libraries/System.Private.Xml/src/System/Xml/Xsl/Runtime/XsltFunctions.cs

@@ -366,7 +366,7 @@ public static string MSFormatDateTime(string dateTime, string format, string lan
         {
             try
             {
-                string locale = GetCultureInfo(lang).Name;
+                CultureInfo ci = GetCultureInfo(lang);
 
                 XsdDateTime xdt;
                 if (!XsdDateTime.TryParse(dateTime, XsdDateTimeFlags.AllXsd | XsdDateTimeFlags.XdrDateTime | XsdDateTimeFlags.XdrTimeNoTz, out xdt))
@@ -375,8 +375,13 @@ public static string MSFormatDateTime(string dateTime, string format, string lan
                 }
                 DateTime dt = xdt.ToZulu();
 
-                // If format is the empty string or not specified, use the default format for the given locale
-                return dt.ToString(format.Length != 0 ? format : null, new CultureInfo(locale));
+                // If format is the empty string or not specified, use the default date or time format for the given locale
+                if (format.Length == 0)
+                {
+                    format = isDate ? "d" : "T";
+                }
+
+                return dt.ToString(format, ci);
             }
             catch (ArgumentException)
             { // Operations with DateTime can throw this exception eventualy

src/libraries/System.Private.Xml/tests/Xslt/XslCompiledTransformApi/XslCompilerTests.cs

@@ -1,7 +1,9 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Globalization;
 using System.IO;
+using System.Tests;
 using System.Xml.Xsl;
 using Xunit;
 
@@ -44,5 +46,71 @@ public void ValueOfInDebugMode()
                 Assert.Equal("This is my class info", outWriter.ToString());
             }
         }
+
+        [Fact]
+        public void FormatDateWithEmptyFormatString()
+        {
+            const string Xml = @"<?xml version=""1.0"" encoding=""UTF-8""?><root/>";
+            const string Xsl = @"<xsl:stylesheet version=""1.0"" xmlns:xsl=""http://www.w3.org/1999/XSL/Transform""
+                    xmlns:ms=""urn:schemas-microsoft-com:xslt"">
+    <xsl:output method=""text"" />
+    <xsl:template match=""/"">
+        <xsl:value-of select=""ms:format-date('2001-02-03T01:02:03', '')"" />
+    </xsl:template>
+</xsl:stylesheet>";
+
+            using (new ThreadCultureChange("en-GB"))
+            using (var outWriter = new StringWriter())
+            {
+                using (var xslStringReader = new StringReader(Xsl))
+                using (var xmlStringReader = new StringReader(Xml))
+                using (var xslReader = XmlReader.Create(xslStringReader))
+                using (var xmlReader = XmlReader.Create(xmlStringReader))
+                {
+                    var transform = new XslCompiledTransform();
+                    transform.Load(xslReader);
+                    transform.Transform(xmlReader, null, outWriter);
+                }
+
+                string result = outWriter.ToString();
+                DateTime expectedDate = new DateTime(2001, 2, 3, 1, 2, 3, DateTimeKind.Utc);
+                string expectedResult = expectedDate.ToString("d", new CultureInfo("en-GB"));
+                Assert.Equal(expectedResult, result);
+            }
+        }
+
+        [Fact]
+        public void FormatTimeWithEmptyFormatString()
+        {
+            const string Xml = @"<?xml version=""1.0"" encoding=""UTF-8""?><root/>";
+            const string Xsl = @"<xsl:stylesheet version=""1.0"" xmlns:xsl=""http://www.w3.org/1999/XSL/Transform""
+                    xmlns:ms=""urn:schemas-microsoft-com:xslt"">
+    <xsl:output method=""text"" />
+    <xsl:template match=""/"">
+        <xsl:value-of select=""ms:format-time('2001-02-03T01:02:03', '')"" />
+    </xsl:template>
+</xsl:stylesheet>";
+
+            using (new ThreadCultureChange("en-GB"))
+            using (var outWriter = new StringWriter())
+            {
+                using (var xslStringReader = new StringReader(Xsl))
+                using (var xmlStringReader = new StringReader(Xml))
+                using (var xslReader = XmlReader.Create(xslStringReader))
+                using (var xmlReader = XmlReader.Create(xmlStringReader))
+                {
+                    var transform = new XslCompiledTransform();
+                    transform.Load(xslReader);
+                    transform.Transform(xmlReader, null, outWriter);
+                }
+
+                string result = outWriter.ToString();
+                DateTime expectedTime = new DateTime(2001, 2, 3, 1, 2, 3, DateTimeKind.Utc);
+                string expectedResult = expectedTime.ToString("T", new CultureInfo("en-GB"));
+                Assert.Equal(expectedResult, result);
+            }
+        }
+
+
     }
 }