The selection controls allow you to navigate through or to change the active selection. For their use, see again Section 1.5.16.
Assume that you have made a selection and that you want to add a time interval of 1 second to the begin of it. In that case you should perform the following actions:
Move the crosshair to the begin of the active selection
Enable Selection Mode
Go back one second by clicking the corresponding button from the media controls.
Turn off the selection mode and enter an annotation for the selection.