fix bug

hutool-core/src/test/java/cn/hutool/core/date/Issue3011Test.java

@@ -0,0 +1,23 @@
+package cn.hutool.core.date;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.Calendar;
+
+public class Issue3011Test {
+	@Test
+	public void isSameMonthTest() {
+		// https://github.com/dromara/hutool/issues/3011
+		// 判断是否同一个月，还需考虑公元前和公元后的的情况
+		// 此处公元前2020年和公元2021年返回年都是2021
+		final Calendar calendar1 = Calendar.getInstance();
+		calendar1.set(-2020, Calendar.FEBRUARY, 12);
+
+		final Calendar calendar2 = Calendar.getInstance();
+		calendar2.set(2021, Calendar.FEBRUARY, 12);
+
+
+		Assert.assertFalse(DateUtil.isSameMonth(calendar1, calendar2));
+	}
+}