fix issue IDFVKG

hutool-core/src/main/java/cn/hutool/core/date/DateBetween.java

@@ -74,13 +74,17 @@ public class DateBetween implements Serializable {
 		Assert.notNull(begin, "Begin date is null !");
 		Assert.notNull(end, "End date is null !");
 
+		// defensive copy
+		Date b = new Date(begin.getTime());
+		Date e = new Date(end.getTime());
+
 		if (isAbs && begin.after(end)) {
 			// 间隔只为正数的情况下，如果开始日期晚于结束日期，置换之
-			this.begin = end;
-			this.end = begin;
+			this.begin = e;
+			this.end = b;
 		} else {
-			this.begin = begin;
-			this.end = end;
+			this.begin = b;
+			this.end = e;
 		}
 	}
 

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

@@ -96,4 +96,17 @@ public class DateBetweenTest {
 		long result = DateUtil.betweenYear(sdate, edate, false);
 		assertEquals(0, result);
 	}
+
+	@Test
+	public void issueIDFVKGTest() {
+		Date b = new Date(1609459200000L); // 2021-01-01 00:00:00
+		Date e = new Date(1609545600000L); // 2021-01-02 00:00:00
+		DateBetween db = new DateBetween(b, e);
+
+		// 修改原始 date
+		b.setTime(0L); // 1970-01-01
+
+		// 期望 DateBetween 不受影响，间隔仍为 1 天
+		assertEquals(1, db.between(DateUnit.DAY));
+	}
 }