As you drag retime markers to adjust the timing of animation, you can force retime markers to snap to whole frames.
To make retime markers snap to frames
- In the Graph Editor, select Transformation Tools > Retime Tool >
(or double-click the Retime Tool icon 
if it displays in the Tool Box area). - In the Retime Settings, turn on Snap Markers on Frame. Note: The Retime Settings display in the Tool Settings window, outside of the Graph Editor.