Polish 'Allow other "timestamp" types in MVC error model'

See gh-23256

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/error/ErrorMvcAutoConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,7 +17,6 @@
 package org.springframework.boot.autoconfigure.web.servlet.error;
 
 import java.nio.charset.StandardCharsets;
-import java.util.Date;
 import java.util.Map;
 import java.util.stream.Collectors;
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/error/ErrorMvcAutoConfigurationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,9 @@
 
 package org.springframework.boot.autoconfigure.web.servlet.error;
 
+import java.time.Clock;
+import java.util.Map;
+
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 
@@ -62,6 +65,22 @@ class ErrorMvcAutoConfigurationTests {
 		});
 	}
 
+	@Test
+	void renderCanUseJavaTimeTypeAsTimestamp() throws Exception { // gh-23256
+		this.contextRunner.run((context) -> {
+			View errorView = context.getBean("error", View.class);
+			ErrorAttributes errorAttributes = context.getBean(ErrorAttributes.class);
+			DispatcherServletWebRequest webRequest = createWebRequest(new IllegalStateException("Exception message"),
+					false);
+			Map<String, Object> attributes = errorAttributes.getErrorAttributes(webRequest, true);
+			attributes.put("timestamp", Clock.systemUTC().instant());
+			errorView.render(attributes, webRequest.getRequest(), webRequest.getResponse());
+			assertThat(webRequest.getResponse().getContentType()).isEqualTo("text/html;charset=UTF-8");
+			String responseString = ((MockHttpServletResponse) webRequest.getResponse()).getContentAsString();
+			assertThat(responseString).contains("This application has no explicit mapping for /error");
+		});
+	}
+
 	@Test
 	void renderWhenAlreadyCommittedLogsMessage(CapturedOutput output) {
 		this.contextRunner.run((context) -> {