Or use 2 layers, with the top layer cropped.
Or make a black and white mask image in Photoshop and apply it to the top layer.
The benifit of this black and white image is that you can use greytones as feather for the masked area if needed, while the spilt and crop options mosty just created straight lines on the edges.
Or use Colorista 3, although that is a 3rd party plugin. It allows to keyframe masks.
LOL,
I now realise that I interpeted it as adjust a part of the image instead of a certain section in time.
If it is a certain part on the timeline: you can use keyframes, or cut that part loose to add the effects. If the transition needs to be smooth, you can add crossfades.