Main toolbar > Curve Editor (Open) > Mode > Dope Sheet > Right-click the blank area of the toolbar to the right of Modify Child Keys. > Show toolbar > Extras: Dope Sheet toolbar

Provides extra tools for use in Dope Sheet mode.

Exclude Left End Point—Prevents a key at the first frame of a selected block of time from being copied to the clipboard.

Exclude Right End Point—Prevents a key at the last frame of a selected block of time from being copied to the clipboard.