I got my hands on Netflix and Apple's delivery specs, and saw some disturbing requirements regarding caption files time bases:
Apple: "The timecode framerate can only be 29.97. This framerate is independent from your video source framerate."
Netflix: All 23.976 fps content shall have subtitles that conform to SMPTE 24 time code. Netflix does not accept subtitles timed to 23.976 video playback rate.
The way to do this is to create a proxy with burned-in time code that is either 29.97 drop frame or SMPTE 24, starting at hour 00. Use that proxy and align the subtitles with the burned-in time code. It may be helpful if you play back 23.976 video at 24.00 fps.
From what I know, Amazon (for example) has no such shenanigans. They accept caption files at 23.976.
Just when I thought 23.976fps was a highly established industry standard for digital film content...
1. From a video engineering perspective, who accepts 23.976fps material and yet wants a synchronized, secondary asset to have a different frame rate?
2. The Apple requirement is, at least, fairly straightforward. But can you help me with Netflix? 23.976fps material already uses 24-base timecode. Are they trying to define some theoretical "drop-frame 23.976" time base? It "may" be helpful to change the video frame rate, based on my mood? Do they want the subtitles sped up by 0.1% or not?
3. Have you ever delivered closed-captions to Netflix or Apple for 23.976fps material? Ideally you could shed some light on how this is handled in practice. Also, these specs are from 2013 and 2014. Maybe things changed?
4. Netflix does accept SRT files, so that may be a way to avoid all that, since those files time in milliseconds and don't use frames at all.
I haven't had to deal with this yet in person (the distributors I deal with are middlemen between my suite and the Netflixes of the world), but this would be good to understand in advance.
PS This has nothing to do with Premiere Pro, but PPro is my favorite NLE these days, and after researching the Cow, 90%+ of closed captioning questions fall into the various NLE forums.