The mtime (modification time) on the directory itself changes when a file or a subdirectory is added, removed or renamed.
Modifying the contents of a file within the directory does not change the directory itself, nor does updating the modified times of a file or a subdirectory. Additionally, adding, removing or renaming files/directories in subdirectories does not propagate up to the directory. If you change the permissions on the directory, the ctime changes but the mtime does not.
目录的mtime改变，当文件或子目录被添加、移除或者重命名时。
修改文件的内容，不会导致目录的mtime发生改变。
目录mtime改变时，不会progagate到父目录。
改变目录的权限时，ctime变化，但是mtime不变
参考
https://stackoverflow.com/questions/3620684/directory-last-modified-date
https://www.baeldung.com/linux/directory-last-modified-time