/usr/bin/time: not the command you think you know
The reason 'time' is a builtin is because it can time a complete shell pipeline, not just a single command. If you type<p><pre><code>    time foo |bar
</code></pre>
Then the result is the total time taken by foo and bar together. This requires it to have special-case syntax. Whereas<p><pre><code>    /usr/bin/time foo |bar
</code></pre>
Would run foo and give its time statistics as input to bar.