title: What does the at-sign @ mean in the bash command: date --date @...?
body: <p>Searching the internet I found explanations only for '$@', meaning 'expand to positional parameters'. But I couldn't find anything about the @ sign by itself.</p>
  
  <p>I stumbled over it in the third snipped of the accepted answer to this question:
  <a href="https://superuser.com/questions/611538/is-there-a-way-to-display-a-countdown-or-stopwatch-timer-in-a-terminal">https://superuser.com/questions/611538/is-there-a-way-to-display-a-countdown-or-stopwatch-timer-in-a-terminal</a></p>
  
  <p>Specifically:</p>
  
  <pre><code>date -u --date @$((`date +%s` - $date1)) +%H:%M:%S
  </code></pre>