Tweaks to [http://bl.ocks.org/mbostock/6232537](mbostock/6232537) that adds data-driven timespans to the widget. This version allows timespans of varying sizes. It also modifies the `brushend` listener to log information based on the timespans where the brush's endpoints are located.

This brush snaps to day boundaries. When the user releases the brush, the brush fires a *brushend* event, allowing a listener to adjust the brush extent. Using the new [brush transitions](/mbostock/6216724) released in [D3 3.3](https://github.com/mbostock/d3/releases/v3.3.0), the brush smoothly interpolates from the original extent to the rounded extent. Compare this approach to using [immediate snapping while brushing](/mbostock/6232620).