Some animation formats (for example FLI/FLC) only support one timecode for all of the frames of an animation, that means that all frames will stop playback for the same amount of time. These animations often have dummy frames inserted into the animation to allow for better timing.
If your project does not have a global timecode or does support a timecode for each frame, this menu item will be disabled. If it does support a timecode for each frame, you can select the frames you want to change the timecode of and select the Local Timecodes menu item of the Frames menu.