In NTSC countries, 24p footage actually runs at 23.976 fps. 23.976 drop-frame timecode is not part of the SMPTE timecode spec and most editing programs don’t have a 24p drop-frame option. There are good reasons for this. Let’s think about how drop-frame timecode works for 29.97. Television programs commonly run in 30 minute increments, so drop-frame timecode should work in such a way that it is guaranteed to be a correct representation of the running time at least every 30 minutes. In fact, drop-frame is cyclical every ten minutes and, therefore, is guaranteed to accurately represent run-time every ten minutes.
At 30 frames per second, to convert ten minutes of programming to a frame count, you multiply:
30 (frames per second) * 60 (seconds per minute) * 10 (minutes) = 18,000
However, since NTSC video operates at 29.97, not 30 frames per second, the actual number of frames in ten minutes of NTSC video is:
29.97 (frames per second) * 60 (seconds per minute) * 10 (minutes) = 17,982
That means that every ten minutes, drop-frame timecode needs to make up for the discrepancy between 18,000 frames and 17,982 frames, which is 18 frames. And that’s exactly what drop-frame timecode does. By skipping two frames every minute, except for minutes divisible by ten, the timecode skips exactly 18 frames every ten minutes and therefore, even if drop-frame timecode may deviate from actual running time by a frame or two within a ten minute cycle, at the end of that cycle, it is guaranteed to be consistent.
Now let’s run those same numbers for 23.976:
24 (frames per second) * 60 (seconds per minute) * 10 (minutes) = 14,400
23.976 (frames per second) * 60 (seconds per minute) * 10 (minutes) = 14,385.6
Already, we can see that we might be in trouble here. The discrepancy over ten minutes is 14.4 frames, which has a decimal component. Even at 30 minutes or 60 minutes, the discrepancy between 24 and 23.976 has a decimal. That means that no matter what rules you set for dropping frames, there’s no way to make it come out evenly over a 60 minute period. There’s actually no way to make it come out evenly even over a 24 hour period. There’s simply no way to devise a formula to accurately do drop-frame for 23.976 footage.