Moment has quite a nice fluent interface for some operations, others just need a little more thought.
For example, I wanted the duration of something and I had recorded the start and end time. I thought something like this, finding the difference between two dates and converting it to a duration, would work:
var duration = endTime.diff(startTime).duration().asSeconds();
However, that doesn’t work.
What you have to do is find the difference, then pass that into the duration function, like this:
var duration = moment.duration(endTime.diff(startTime)).asSeconds();
And now I get what I wanted.