diff --git a/src/NSwag.CodeGeneration.CSharp/Templates/Client.Class.HeaderParameter.liquid b/src/NSwag.CodeGeneration.CSharp/Templates/Client.Class.HeaderParameter.liquid
index 9d686c515..571aa4e38 100644
--- a/src/NSwag.CodeGeneration.CSharp/Templates/Client.Class.HeaderParameter.liquid
+++ b/src/NSwag.CodeGeneration.CSharp/Templates/Client.Class.HeaderParameter.liquid
@@ -1,7 +1,9 @@
 {% if parameter.IsStringArray -%}
 request_.Headers.TryAddWithoutValidation("{{ parameter.Name }}", {{ parameter.VariableName }});
 {% elsif parameter.IsDateTime -%}
-request_.Headers.TryAddWithoutValidation("{{ parameter.Name }}", ConvertToString({{ parameter.VariableName }}{% if parameter.IsNullable %}?{% endif %}.ToString("{{ ParameterDateTimeFormat }}"), System.Globalization.CultureInfo.InvariantCulture));
+request_.Headers.TryAddWithoutValidation("{{ parameter.Name }}", ConvertToString({{ parameter.VariableName }}{% if parameter.IsNullable or parameter.IsSystemNullable %}?{% endif %}.ToString("{{ ParameterDateTimeFormat }}"), System.Globalization.CultureInfo.InvariantCulture));
+{% elsif parameter.IsDate -%}
+request_.Headers.TryAddWithoutValidation("{{ parameter.Name }}", ConvertToString({{ parameter.VariableName }}{% if parameter.IsNullable or parameter.IsSystemNullable %}?{% endif %}.ToString("{{ ParameterDateFormat }}"), System.Globalization.CultureInfo.InvariantCulture));
 {% else -%}
 request_.Headers.TryAddWithoutValidation("{{ parameter.Name }}", ConvertToString({{ parameter.VariableName }}, System.Globalization.CultureInfo.InvariantCulture));
 {%- endif %}