launchd run command every minute of a specific hour
Edit start

You got it all correct except for enclosing your &lt;dict&gt; element inside an &lt;array&gt; element. Your script will run every minute from 21:00 to 21:59 with the following as your StartCalendarInterval key : 

    &lt;key&gt;StartCalendarInterval&lt;/key&gt;
    &lt;array&gt;
        &lt;dict&gt;
            &lt;key&gt;Hour&lt;/key&gt;
            &lt;integer&gt;21&lt;/integer&gt;
        &lt;/dict&gt;
    &lt;/array&gt;


If you wanted your script to run every minute, every hour, every day, every year, the following would be the correct (but certainly not obvious)  syntax for that: 

    &lt;key&gt;StartCalendarInterval&lt;/key&gt;
    &lt;array&gt;
        &lt;dict/&gt;
    &lt;/array&gt;



My original answer (below) would also work, but it's clearly much more tedious! OTOH, if one needed every other minute, or certain minutes, it might be useful.

Edit end



&lt;key&gt;StartCalendarInterval&lt;/key&gt;
    &lt;array&gt;
        &lt;dict&gt;
            &lt;key&gt;Minute&lt;/key&gt;
            &lt;integer&gt;0&lt;/integer&gt;
        &lt;/dict&gt;
        &lt;dict&gt;
            &lt;key&gt;Minute&lt;/key&gt;
            &lt;integer&gt;1&lt;/integer&gt;
        &lt;/dict&gt;
        &lt;dict&gt;
            &lt;key&gt;Minute&lt;/key&gt;
            &lt;integer&gt;2&lt;/integer&gt;
        &lt;/dict&gt;
        &lt;dict&gt;
            &lt;key&gt;Minute&lt;/key&gt;
            &lt;integer&gt;3&lt;/integer&gt;
        &lt;/dict&gt;
        &lt;dict&gt;
            &lt;key&gt;Minute&lt;/key&gt;
            &lt;integer&gt;4&lt;/integer&gt;
        &lt;/dict&gt;

       ... ad nauseum ...

        &lt;dict&gt;
            &lt;key&gt;Minute&lt;/key&gt;
            &lt;integer&gt;59&lt;/integer&gt;
        &lt;/dict&gt;

    &lt;/array&gt;


To answer your question, "Why is this not running every minute?", it's only because you failed to enclose &lt;dict&gt; inside &lt;array&gt;.  FWIW, it seems very odd to me also, and WHY it's designed this way is an answer I'd like to hear myself. But then I guess Apple could use the defense that "just because the semantics are similar does not mean the syntax is."