When multiple images are selected, Lightroom needs to know which one it should use when you take an action that can apply to only one of them (such as switching to Develop, i.e. which of the selected images should be loaded into the Develop Loupe window?). To deal with this it has the concept that one of the multiple selections will always be regarded as the "most selected"....try selecting a bunch of images then look carefully at the frames in the Grid or Filmstrip, they'll all be a lighter grey than the standard "not selected" colour, but one of them will be lighter still. That's the "most selected" image, and you can change any of the selected images to be the "most selected" by single-clicking on it.
So, to change the capture time of a bunch of selected images, make the selection, then use the "Edit Capture Time" dialog to enter the new time for just the "most selected" image, and all the other images in the selection will have their capture times adjusted by the same amount as the "most selected" image.