Trying to understand the modification time

Hi,

I'm investigating a folder named Backups.backupdb on an external hard drive.
I assume the folder was created by Time Machine.

Running the "ls -l" command, the time displayed for Backups.backupdb is "June 1".
I believe that this is the modification time (mtime).

What I can not (or hardly) understand is that within subfolders of Backups.backupdb are some backups dated "June 2".

Is is normal that the modification time of a parent folder does not change when its content change ?

Thanks.

Within the backup folder that Time Machine creates should be a long list of the individual backups that it made after the first initial backup. So yes, the date of the initial backup or the creation date of the backup folder does not change with subsequent backups, only the individual backup dates will change as they are made.

Thanks.

Yes, there are several backup folders. What I find suprising is not the fact that the first backups keep their dates, as they're not altered by subsequent backups, but the fact that the "modification time" of the parent folder is not updated as it gets new content, i.e. a new subfolder, at each backup. Maybe was this chosen for OS performance reasons...