To install:

First download a .zip of this code. Then, open Blender, `Edit -> Preferences -> Add-ons -> Install`, select the .zip. It's now installed. To enable, check the box next to `Animation: Retime` so it's checkmarked. Preferences can be set here.

If any issues are found, please contact me.

To use:

• Select some keyframes, and choose `Retime Keyframes` in the `Key` menu of the Dopesheet Editor or Graph Editor.
• Or, select some markers, and choose `Retime Markers` in the `Markers` menu of the Dopesheet Editor or Graph Editor.
  

Now, move your mouse or type the desired frame count to retime the keyframes within the range.

The operator works in 'Nearest Frame', 'Frame Step', and 'No Auto-Snap' modes. Holding 'Ctrl' will toggle snapping to Nearest Frame.

Version History:

1.0.0: It works.

1.0.1: It works better.

1.0.2: Timeline and pose markers can be used to retime keyframes.