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`. The addon will enable, and Preferences can be set there.

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.
• Optionally, pressing 'N' while mousing over the above editors will toggle the sidebar, where you can access the 'Retiming' tab to filter which types of Keyframes the addon will affect. You can also quickly change addon preferences.
  

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 disable all auto-snap and allow the creation of "Partial Keyframes". (Note that, as Grease Pencil Keyframes are always snapped to Nearest Frame, this 'Ctrl' functionality will be disabled if any are selected.)

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.

1.1.0:

• Added: Blender 4.X support (Thanks to Pluglug for their contributions!)
• Added: Shape Key support.
• Added: Grease Pencil support (As grease pencil frames are always integers, Partial Keyframe support is disabled when the selection includes them).
• Added: Panel for filtering which types of keyframes (Object, Grease Pencil, Shape Key) will be affected by retiming.
• Added: Panel for easily toggling of addon preferences. (Useful in conjunction with filtering)
• Added: Error message display when keyframes will be overwritten.
• Added: Retime Markers option to the Timeline editor's Marker menu.
• Changed: Header now shows the retiming duration before and after, along with the percentage change.
• Changed: Footer now shows operator modifier keys, as similar to builtin blender operators as possible with the current Blender API (no icons, sadly).
• Changed: Operator now skips processing objects that have no animation data, which should improve performance slightly.
• Changed: Error Message displays when no keyframes are selected, now in all situations.