修复FastDatePrinter处理YY错误问题

hutool-core/src/main/java/org/dromara/hutool/core/date/format/FastDatePrinter.java

@@ -967,7 +967,12 @@ public class FastDatePrinter extends SimpleDateBasic implements DatePrinter {
 
 		@Override
 		public void appendTo(final Appendable buffer, final Calendar calendar) throws IOException {
-			mRule.appendTo(buffer, calendar.getWeekYear());
+			int weekYear = calendar.getWeekYear();
+			if (mRule instanceof TwoDigitYearField) {
+				// issue#3641
+				weekYear %= 100;
+			}
+			mRule.appendTo(buffer, weekYear);
 		}
 
 		@Override

hutool-core/src/test/java/org/dromara/hutool/core/date/format/FastDateFormatTest.java

@@ -0,0 +1,58 @@
+package org.dromara.hutool.core.date.format;
+
+import org.dromara.hutool.core.date.DateUtil;
+import org.junit.jupiter.api.Test;
+
+import java.text.SimpleDateFormat;
+import java.util.Date;
+import java.util.TimeZone;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class FastDateFormatTest {
+	private static final TimeZone timezone = TimeZone.getTimeZone("Etc/Utc");
+
+	private static FastDateFormat getHutoolInstance(final String pattern) {
+		return FastDateFormat.getInstance(pattern, timezone);
+	}
+
+	@Test
+	public void yearTest() {
+		final Date date = DateUtil.date(0L);
+
+		assertEquals(
+			"1970-01-01 00:00:00",
+			getHutoolInstance("yyyy-MM-dd HH:mm:ss").format(date)
+		);
+
+		assertEquals(
+			"1970-01-01 00:00:00",
+			getHutoolInstance("YYYY-MM-dd HH:mm:ss").format(date)
+		);
+
+		assertEquals(
+			"1970",
+			getHutoolInstance("YYYY").format(date)
+		);
+
+		assertEquals(
+			"70",
+			getHutoolInstance("yy").format(date)
+		);
+	}
+
+	@Test
+	public void weekYearTest() {
+		final Date date = DateUtil.date(0L);
+
+		assertEquals(
+			"70",
+			new SimpleDateFormat("YY").format(date)
+		);
+
+		assertEquals(
+			"70",
+			getHutoolInstance("YY").format(date)
+		);
+	}
+}