What does the '@' mean before a bash variable?

I’m working on a bash script that checks if a backup has been made in the last X days. The script must be run on linux and macos. My problem is, that when it checks if it’s running on a linux or macos system, I don’t understand what the ‘@’ means before the ‘bckpT’ variable on linux part.

case "$OS" in
   DARWIN)
       lastBackup=$($DATE -u -r $bckpT +%F)
       ;;

   LINUX)
       lastBackup=$($DATE -d @$bckpT +%F)
       ;;

   WIN)
       $ECHO "No script for WIN"
       ;;

   *)
       help
       ;;
esac

When there isn’t the ‘@’, the output is:

/usr/bin/date: invalid date ‘1647907200’

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>Solution :

In date command the @ sign tells the command to treat the following as seconds since epoch

In your case, it consider the param $bckpT value as seconds since epoch