In the $integral and the $derivative window functions, 'outputUnit' specifies how to express time deltas as a number. We originally called it 'outputUnit' to avoid a name collision with the 'unit' used for the window bounds.

This name is confusing though, because often the measurements implicitly have a unit involving time. For example, suppose you have numbers representing meters/second, and times represented as a datetime. The result will have units of meters, so writing outputUnit: 'second' is counterintuitive.

Now that the 'window' arguments are separate, we could go back to calling this argument just 'unit':

{$integral: {
   input: "$rate",
   unit: "second"
 },
 window: {
   range: [-1, 0],
   unit: "hour"
 }
}