About Time Machine local snapshots on Mac
Time Machine makes copies, each hour, of files that have changed. These local snapshots are stored on the same disk as the original files, and are separate from backups stored on your backup disk. These files are stored for up to 24 hours or until space is needed on the disk. When your normal backup disk isn’t available, local snapshots are a convenient way to retrieve earlier versions of your files. It is recommended that you always perform normal backups in case of a disaster.
Note: The date of the latest local snapshot is not reflected in the latest backup dates shown in Time Machine Settings.
Important: Local snapshots are only stored on disks using Apple File System (APFS) when Back Up Automatically is selected in Time Machine settings.
On your Mac, choose Apple menu 
. Time Machine automatically removes local snapshots when space is needed on the disk or when they are older than 24 hours.