It keeps exactly one active (unarchived) backup per device, until you perform a restore, or you choose to delete it.
From Create and delete iPhone, iPad, and iPod touch backups in iTunes
Only app data from App Store apps and sync settings will be restored to your device when you:
- Restore your iPad from an iPhone or iPod touch backup.
- Restore your iPhone or iPod touch from an iPad backup.
iTunes creates an archived, time-stamped backup when you:
- Restore from backup.
- Erase or restore a device you previously backed up and choose to set up as new in iTunes. If you use a different iTunes library (another user account or another computer) and select Setup as New or Restore from an existing backup, iTunes overrides the non-archived backup the next time you connect the device to your primary iTunes library and sync automatically is enabled.
- Right-click a backup, and select Archive. (Available in iTunes for Mac only.)
You may have older backups from previous restores or device upgrades, but there is still only one active backup. Unless you restored the device after backing it up yesterday, then today's backup simply updated the active backup for the device, which is why you don't see anything with yesterday's date on it.
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