Changelog area shows changes, which you made on timeline. You can see exactly what you've changed. If you want you can add a comment to your changes. After clicking Save comments field will show. Comment will be saved on every issue from changelog.
You can revert any change you want by clicking revert on the left side of every change. To save your changes just click Save button. You will see that Status of every issue changed to Success and Changelog was cleared. If one of the changes can't be saved its Status will change to Error and error message will be shown.
You can edit date and time in Changelog. Just click the pencil icon and the pop-up calendar will appear.
Notice that operations changelog depends on the configuration (v.1.4.0).
In example below you have selected Drag end and on changelog you can change only End date.