diff --git a/appveyor.yml b/appveyor.yml
index 7ac8c6b..b65ae52 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,7 +1,7 @@
 version: build-{build}
 skip_tags: true
 image: Visual Studio 2019
-configuration: Release
+configuration: Debug
 build_script:
 - ps: |-
     #cd src
diff --git a/src/MiniExcel/Utils/DateTimeHelper.cs b/src/MiniExcel/Utils/DateTimeHelper.cs
index 05a701e..276ee22 100644
--- a/src/MiniExcel/Utils/DateTimeHelper.cs
+++ b/src/MiniExcel/Utils/DateTimeHelper.cs
@@ -6,8 +6,6 @@
 
     internal static partial class DateTimeHelper
     {
-        private static DateTime basicDate = new DateTime(2021, 01, 01);
-
         /// <summary>
         /// NumberFormat from NuGet ExcelNumberFormat MIT@License
         /// </summary>
@@ -15,17 +13,6 @@
         {
             return new NumberFormat(formatCode).IsDateTimeFormat;
         }
-
-        public static bool IsSingleDatetimeFormat(string formatCode)
-        {
-            var isDatetimeFormat = DateTime.TryParseExact(basicDate.ToString(formatCode), formatCode, CultureInfo.InvariantCulture, DateTimeStyles.AllowLeadingWhite | DateTimeStyles.AllowTrailingWhite, out var date);
-            if (basicDate != date)
-                isDatetimeFormat = false;
-
-            // TODO: double check
-            // TODO: datetime format like yyyy need to convert to string not datetime
-            return isDatetimeFormat;
-        }
     }
 
     internal static partial class DateTimeHelper
diff --git a/tests/MiniExcelTests/MiniExcelCsvAsycTests.cs b/tests/MiniExcelTests/MiniExcelCsvAsycTests.cs
index a88cd17..65bea79 100644
--- a/tests/MiniExcelTests/MiniExcelCsvAsycTests.cs
+++ b/tests/MiniExcelTests/MiniExcelCsvAsycTests.cs
@@ -15,7 +15,7 @@ namespace MiniExcelLibs.Tests
     public class MiniExcelCsvAsycTests
     {
         [Fact]
-        public async Task gb2312_Encoding_Read_Test()
+        public async Task Gb2312_Encoding_Read_Test()
         {
             System.Text.Encoding.RegisterProvider(System.Text.CodePagesEncodingProvider.Instance);
             var path = PathHelper.GetSamplePath("csv/gb2312_Encoding_Read_Test.csv");
@@ -183,7 +183,7 @@ namespace MiniExcelLibs.Tests
         public async Task CsvExcelTypeTest()
         {
             {
-                var path = Path.Combine(Path.GetTempPath(), $"{Guid.NewGuid().ToString()}.csv");
+                var path = Path.Combine(Path.GetTempPath(), $"{Guid.NewGuid()}.csv");
                 var input = new[] { new { A = "Test1", B = "Test2" } };
                 await MiniExcel.SaveAsAsync(path, input);
 
@@ -215,7 +215,7 @@ namespace MiniExcelLibs.Tests
         [Fact()]
         public async Task Create2x2_Test()
         {
-            var path = Path.Combine(Path.GetTempPath(), $"{Guid.NewGuid().ToString()}.csv");
+            var path = Path.Combine(Path.GetTempPath(), $"{Guid.NewGuid()}.csv");
             await MiniExcel.SaveAsAsync(path, new[] {
              new { c1 = "A1" ,c2 = "B1"},
              new { c1 = "A2" ,c2 = "B2"},
@@ -244,7 +244,7 @@ namespace MiniExcelLibs.Tests
         [Fact()]
         public async Task CsvTypeMappingTest()
         {
-            var path = Path.Combine(Path.GetTempPath(), $"{Guid.NewGuid().ToString()}.csv");
+            var path = Path.Combine(Path.GetTempPath(), $"{Guid.NewGuid()}.csv");
             await MiniExcel.SaveAsAsync(path, new[] {
                 new { c1 = "A1" ,c2 = "B1"},
                 new { c1 = "A2" ,c2 = "B2"},