Avoid duplicate date-related converters registration

Prior to this commit, `DefaultFormattingConversionService` would
register converters with both `DateTimeFormatterRegistrar` and
`DateFormatterRegistrar`, the former also registering the legace date
converters that the latter contributes.

While we cannot change the behavior for `DateTimeFormatterRegistrar` or
`DateFormatterRegistrar` because of their public contract, we can update
the `DefaultFormattingConversionService` to not use
`DateFormatterRegistrar` and register manually the annotation support
that it contributes.

Closes gh-36951

spring-context/src/main/java/org/springframework/format/support/DefaultFormattingConversionService.java

@@ -20,7 +20,7 @@ import org.jspecify.annotations.Nullable;
 
 import org.springframework.core.convert.support.DefaultConversionService;
 import org.springframework.format.FormatterRegistry;
-import org.springframework.format.datetime.DateFormatterRegistrar;
+import org.springframework.format.datetime.DateTimeFormatAnnotationFormatterFactory;
 import org.springframework.format.datetime.standard.DateTimeFormatterRegistrar;
 import org.springframework.format.number.NumberFormatAnnotationFormatterFactory;
 import org.springframework.format.number.money.CurrencyUnitFormatter;
@@ -115,11 +115,12 @@ public class DefaultFormattingConversionService extends FormattingConversionServ
 
 		// Default handling of date-time values
 
-		// just handling JSR-310 specific date and time types
+		// Handling of JSR-310 specific date and time types, along with the legacy
+		// Date/Calendar/Long converters (see DateTimeConverters).
 		new DateTimeFormatterRegistrar().registerFormatters(formatterRegistry);
 
-		// regular DateFormat-based Date, Calendar, Long converters
-		new DateFormatterRegistrar().registerFormatters(formatterRegistry);
+		// Support for the @DateTimeFormat annotation on legacy Date/Calendar fields
+		formatterRegistry.addFormatterForFieldAnnotation(new DateTimeFormatAnnotationFormatterFactory());
 	}
 
 }

spring-context/src/test/java/org/springframework/format/support/FormattingConversionServiceTests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.format.support;
 
+import java.util.Calendar;
+import java.util.Date;
 import java.util.Locale;
 
 import org.junit.jupiter.api.AfterEach;
@@ -30,6 +32,7 @@ import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.converter.ConverterFactory;
 import org.springframework.core.convert.support.DefaultConversionService;
 import org.springframework.format.Formatter;
+import org.springframework.format.annotation.DateTimeFormat;
 import org.springframework.format.number.NumberStyleFormatter;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -176,6 +179,28 @@ class FormattingConversionServiceTests {
 		assertThat(formattingService.convert("1", Integer.class)).isEqualTo(Integer.valueOf(1));
 	}
 
+	@Test  // gh-36951
+	void defaultFormattingConversionServiceRegistersLegacyDateConvertersOnlyOnce() {
+		DefaultFormattingConversionService defaultService = new DefaultFormattingConversionService();
+		String convertersDescription = defaultService.toString();
+		assertThat(convertersDescription)
+				.containsOnlyOnce("DateFormatterRegistrar$DateToLongConverter")
+				.containsOnlyOnce("DateFormatterRegistrar$CalendarToDateConverter");
+	}
+
+	@Test  // gh-36951
+	void defaultFormattingConversionServiceStillAppliesDateTimeFormatAnnotationToLegacyDateAndCalendarFields() throws Exception {
+		DefaultFormattingConversionService defaultService = new DefaultFormattingConversionService();
+		TypeDescriptor dateDescriptor = new TypeDescriptor(AnnotatedDateBean.class.getDeclaredField("date"));
+		TypeDescriptor calendarDescriptor = new TypeDescriptor(AnnotatedDateBean.class.getDeclaredField("calendar"));
+
+		Date date = (Date) defaultService.convert("2026-09-19", TypeDescriptor.valueOf(String.class), dateDescriptor);
+		Calendar calendar = (Calendar) defaultService.convert("2026-09-19", TypeDescriptor.valueOf(String.class), calendarDescriptor);
+
+		assertThat(defaultService.convert(date, dateDescriptor, TypeDescriptor.valueOf(String.class))).isEqualTo("2026-09-19");
+		assertThat(defaultService.convert(calendar, calendarDescriptor, TypeDescriptor.valueOf(String.class))).isEqualTo("2026-09-19");
+	}
+
 
 	static class NullReturningFormatter implements Formatter<Integer> {
 
@@ -214,4 +239,14 @@ class FormattingConversionServiceTests {
 		}
 	}
 
+
+	private static class AnnotatedDateBean {
+
+		@DateTimeFormat(pattern = "yyyy-MM-dd")
+		private Date date;
+
+		@DateTimeFormat(pattern = "yyyy-MM-dd")
+		private Calendar calendar;
+	}
+
 }