diff --git a/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/generichost/DateTimeJsonConverter.mustache b/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/generichost/DateTimeJsonConverter.mustache
index 55ecf4a1660..21d35ec2302 100644
--- a/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/generichost/DateTimeJsonConverter.mustache
+++ b/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/generichost/DateTimeJsonConverter.mustache
@@ -46,6 +46,6 @@ namespace {{packageName}}.{{clientPackage}}
         /// <param name="dateTimeValue"></param>
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, DateTime dateTimeValue, JsonSerializerOptions options) =>
-            writer.WriteStringValue(dateTimeValue.ToString(Formats[0], CultureInfo.InvariantCulture));
+            writer.WriteStringValue(dateTimeValue.ToString("{{{dateTimeFormat}}}", CultureInfo.InvariantCulture));
     }
 }
diff --git a/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/generichost/DateTimeNullableJsonConverter.mustache b/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/generichost/DateTimeNullableJsonConverter.mustache
index f052afa1491..52e2afa0b7a 100644
--- a/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/generichost/DateTimeNullableJsonConverter.mustache
+++ b/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/generichost/DateTimeNullableJsonConverter.mustache
@@ -50,7 +50,7 @@ namespace {{packageName}}.{{clientPackage}}
             if (dateTimeValue == null)
                 writer.WriteNullValue();
             else
-                writer.WriteStringValue(dateTimeValue.Value.ToString(Formats[0], CultureInfo.InvariantCulture));
+                writer.WriteStringValue(dateTimeValue.Value.ToString("{{{dateTimeFormat}}}", CultureInfo.InvariantCulture));
         }
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
index bb0457c6e9d..71992dc8d86 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
@@ -71,6 +71,6 @@ namespace Org.OpenAPITools.Client
         /// <param name="dateTimeValue"></param>
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, DateTime dateTimeValue, JsonSerializerOptions options) =>
-            writer.WriteStringValue(dateTimeValue.ToString(Formats[0], CultureInfo.InvariantCulture));
+            writer.WriteStringValue(dateTimeValue.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
index d3a7b947bc7..90d2174e4a3 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
@@ -75,7 +75,7 @@ namespace Org.OpenAPITools.Client
             if (dateTimeValue == null)
                 writer.WriteNullValue();
             else
-                writer.WriteStringValue(dateTimeValue.Value.ToString(Formats[0], CultureInfo.InvariantCulture));
+                writer.WriteStringValue(dateTimeValue.Value.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
         }
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
index 6222464831d..e8291cb2e1e 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
@@ -71,6 +71,6 @@ namespace Org.OpenAPITools.Client
         /// <param name="dateTimeValue"></param>
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, DateTime dateTimeValue, JsonSerializerOptions options) =>
-            writer.WriteStringValue(dateTimeValue.ToString(Formats[0], CultureInfo.InvariantCulture));
+            writer.WriteStringValue(dateTimeValue.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
index 4e53bbe9557..e26c40071cd 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
@@ -75,7 +75,7 @@ namespace Org.OpenAPITools.Client
             if (dateTimeValue == null)
                 writer.WriteNullValue();
             else
-                writer.WriteStringValue(dateTimeValue.Value.ToString(Formats[0], CultureInfo.InvariantCulture));
+                writer.WriteStringValue(dateTimeValue.Value.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
         }
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-allOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-allOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
index 0083ee7169a..245aaa03ef2 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-allOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-allOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
@@ -71,6 +71,6 @@ namespace Org.OpenAPITools.Client
         /// <param name="dateTimeValue"></param>
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, DateTime dateTimeValue, JsonSerializerOptions options) =>
-            writer.WriteStringValue(dateTimeValue.ToString(Formats[0], CultureInfo.InvariantCulture));
+            writer.WriteStringValue(dateTimeValue.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-allOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-allOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
index 95546974e1e..8f3c26a5869 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-allOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-allOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
@@ -75,7 +75,7 @@ namespace Org.OpenAPITools.Client
             if (dateTimeValue == null)
                 writer.WriteNullValue();
             else
-                writer.WriteStringValue(dateTimeValue.Value.ToString(Formats[0], CultureInfo.InvariantCulture));
+                writer.WriteStringValue(dateTimeValue.Value.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
         }
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-anyOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-anyOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
index ba2d0c137ba..c53e443828d 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-anyOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-anyOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
@@ -71,6 +71,6 @@ namespace Org.OpenAPITools.Client
         /// <param name="dateTimeValue"></param>
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, DateTime dateTimeValue, JsonSerializerOptions options) =>
-            writer.WriteStringValue(dateTimeValue.ToString(Formats[0], CultureInfo.InvariantCulture));
+            writer.WriteStringValue(dateTimeValue.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-anyOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-anyOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
index 1bc64cb2496..fd867d948f1 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-anyOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-anyOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
@@ -75,7 +75,7 @@ namespace Org.OpenAPITools.Client
             if (dateTimeValue == null)
                 writer.WriteNullValue();
             else
-                writer.WriteStringValue(dateTimeValue.Value.ToString(Formats[0], CultureInfo.InvariantCulture));
+                writer.WriteStringValue(dateTimeValue.Value.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
         }
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-oneOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-oneOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
index ba2d0c137ba..c53e443828d 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-oneOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-oneOf/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
@@ -71,6 +71,6 @@ namespace Org.OpenAPITools.Client
         /// <param name="dateTimeValue"></param>
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, DateTime dateTimeValue, JsonSerializerOptions options) =>
-            writer.WriteStringValue(dateTimeValue.ToString(Formats[0], CultureInfo.InvariantCulture));
+            writer.WriteStringValue(dateTimeValue.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-oneOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-oneOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
index 1bc64cb2496..fd867d948f1 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-oneOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netcore-latest-oneOf/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
@@ -75,7 +75,7 @@ namespace Org.OpenAPITools.Client
             if (dateTimeValue == null)
                 writer.WriteNullValue();
             else
-                writer.WriteStringValue(dateTimeValue.Value.ToString(Formats[0], CultureInfo.InvariantCulture));
+                writer.WriteStringValue(dateTimeValue.Value.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
         }
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
index 6222464831d..e8291cb2e1e 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Client/DateTimeJsonConverter.cs
@@ -71,6 +71,6 @@ namespace Org.OpenAPITools.Client
         /// <param name="dateTimeValue"></param>
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, DateTime dateTimeValue, JsonSerializerOptions options) =>
-            writer.WriteStringValue(dateTimeValue.ToString(Formats[0], CultureInfo.InvariantCulture));
+            writer.WriteStringValue(dateTimeValue.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
     }
 }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
index 4e53bbe9557..e26c40071cd 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Client/DateTimeNullableJsonConverter.cs
@@ -75,7 +75,7 @@ namespace Org.OpenAPITools.Client
             if (dateTimeValue == null)
                 writer.WriteNullValue();
             else
-                writer.WriteStringValue(dateTimeValue.Value.ToString(Formats[0], CultureInfo.InvariantCulture));
+                writer.WriteStringValue(dateTimeValue.Value.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK", CultureInfo.InvariantCulture));
         }
     }
 }