I've noticed that for some video sources with a (constant) frame-rate of 29.97 it'll output a video with a duration of 100.034 and 2998 frames instead of the expected 100 and 2997. For subsequent segments (e.g. ffmpeg -ss 100 -t 100) it will output a video with a duration of 100.67 and a framerate of 2999.

I've noticed that for some video sources with a (constant) frame-rate of 29.97 it'll output a video with a duration of 100.034 and 2998 frames instead of the expected 100 and 2997. For subsequent segments (e.g. ffmpeg -ss 100 -t 100) it will output a video with a duration of 100.67 and 2999 frames.

Full command:

ffmpeg -ss 0 -t 100 -i "./input.mov" -map V:0 -c:v libx264 -vf scale=-2:240 out.mp4

Logs:

ffmpeg version 2021-01-12-git-ca21cb1e36-full_build-www.gyan.dev Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 10.2.0 (Rev6, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libglslang --enable-vulkan --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      56. 63.100 / 56. 63.100
  libavcodec     58.116.100 / 58.116.100
  libavformat    58. 65.101 / 58. 65.101
  libavdevice    58. 11.103 / 58. 11.103
  libavfilter     7. 95.100 /  7. 95.100
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
  libpostproc    55.  8.100 / 55.  8.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from './input.mov':
  Metadata:
    major_brand     : qt  
    minor_version   : 512
    compatible_brands: qt  
    encoder         : Lavf58.35.100
  Duration: 00:07:09.83, start: 0.000000, bitrate: 37022 kb/s
    Stream #0:0: Audio: pcm_s16le (sowt / 0x74776F73), 44100 Hz, stereo, s16, 1411 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
      vendor_id       : [0][0][0][0]
    Stream #0:1: Audio: pcm_s16le (sowt / 0x74776F73), 48000 Hz, stereo, s16, 1536 kb/s
    Metadata:
      handler_name    : SoundHandler
      vendor_id       : [0][0][0][0]
    Stream #0:2: Audio: pcm_s16le (sowt / 0x74776F73), 48000 Hz, stereo, s16, 1536 kb/s
    Metadata:
      handler_name    : SoundHandler
      vendor_id       : [0][0][0][0]
    Stream #0:3: Video: prores (HQ) (apch / 0x68637061), yuv422p10le(tv, bt709, progressive), 1920x1080, 32531 kb/s, SAR 1:1 DAR 16:9, 29.97 fps, 29.97 tbr, 30k tbn, 30k tbc (default)
    Metadata:
      handler_name    : VideoHandler
      vendor_id       : FFMP
      encoder         : Lavc58.64.101 prores_ks
Stream mapping:
  Stream #0:3 -> #0:0 (prores (native) -> h264 (libx264))
Press [q] to stop, [?] for help
[libx264 @ 000001a481da7080] using SAR=640/639
[libx264 @ 000001a481da7080] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 000001a481da7080] profile High 4:2:2, level 2.1, 4:2:2, 10-bit
[libx264 @ 000001a481da7080] 264 - core 161 r3033 0d754ec - H.264/MPEG-4 AVC codec - Copyleft 2003-2020 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=7 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=81 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'out.mp4':
  Metadata:
    major_brand     : qt  
    minor_version   : 512
    compatible_brands: qt  
    encoder         : Lavf58.65.101
    Stream #0:0: Video: h264 (avc1 / 0x31637661), yuv422p10le(tv, bt709, progressive), 426x240 [SAR 640:639 DAR 16:9], q=2-31, 29.97 fps, 30k tbn (default)
    Metadata:
      handler_name    : VideoHandler
      vendor_id       : FFMP
      encoder         : Lavc58.116.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
frame=    1 fps=0.0 q=0.0 size=       0kB time=00:00:00.00 bitrate=N/A speed=   0x    
frame=  140 fps=0.0 q=41.0 size=       0kB time=00:00:02.86 bitrate=   0.1kbits/s speed=5.72x    
frame=  273 fps=272 q=41.0 size=       0kB time=00:00:07.30 bitrate=   0.1kbits/s speed=7.28x    
frame=  423 fps=281 q=41.0 size=     256kB time=00:00:12.31 bitrate= 170.4kbits/s speed=8.17x    
frame=  571 fps=284 q=41.0 size=     512kB time=00:00:17.25 bitrate= 243.2kbits/s speed=8.58x    
frame=  715 fps=284 q=41.0 size=     768kB time=00:00:22.05 bitrate= 285.3kbits/s speed=8.77x    
frame=  853 fps=283 q=41.0 size=     768kB time=00:00:26.66 bitrate= 236.0kbits/s speed=8.84x    
frame=  986 fps=280 q=41.0 size=    1024kB time=00:00:31.09 bitrate= 269.8kbits/s speed=8.84x    
frame= 1117 fps=278 q=41.0 size=    1280kB time=00:00:35.46 bitrate= 295.6kbits/s speed=8.82x    
frame= 1263 fps=279 q=41.0 size=    1536kB time=00:00:40.34 bitrate= 311.9kbits/s speed=8.93x    
frame= 1435 fps=286 q=41.0 size=    1536kB time=00:00:46.07 bitrate= 273.1kbits/s speed=9.18x    
frame= 1584 fps=287 q=41.0 size=    1536kB time=00:00:51.05 bitrate= 246.5kbits/s speed=9.25x    
frame= 1734 fps=288 q=41.0 size=    1536kB time=00:00:56.05 bitrate= 224.5kbits/s speed= 9.3x    
frame= 1880 fps=288 q=41.0 size=    1792kB time=00:01:00.92 bitrate= 240.9kbits/s speed=9.34x    
frame= 2013 fps=286 q=41.0 size=    1792kB time=00:01:05.36 bitrate= 224.6kbits/s speed= 9.3x    
frame= 2178 fps=289 q=41.0 size=    1792kB time=00:01:10.87 bitrate= 207.1kbits/s speed=9.41x    
frame= 2338 fps=291 q=41.0 size=    2048kB time=00:01:16.20 bitrate= 220.2kbits/s speed=9.49x    
frame= 2484 fps=291 q=41.0 size=    2048kB time=00:01:21.08 bitrate= 206.9kbits/s speed= 9.5x    
frame= 2646 fps=293 q=41.0 size=    2048kB time=00:01:26.48 bitrate= 194.0kbits/s speed=9.57x    
frame= 2814 fps=295 q=38.0 size=    2304kB time=00:01:32.09 bitrate= 205.0kbits/s speed=9.65x    
frame= 2981 fps=297 q=41.0 size=    2304kB time=00:01:37.66 bitrate= 193.3kbits/s speed=9.72x    
frame= 2998 fps=296 q=-1.0 Lsize=    2533kB time=00:01:39.93 bitrate= 207.7kbits/s speed=9.86x    
video:2498kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.416280%
[libx264 @ 000001a481da7080] frame I:17    Avg QP:28.16  size:  6543
[libx264 @ 000001a481da7080] frame P:849   Avg QP:35.09  size:  2215
[libx264 @ 000001a481da7080] frame B:2132  Avg QP:37.45  size:   265
[libx264 @ 000001a481da7080] consecutive B-frames:  3.6%  3.8%  2.5% 90.1%
[libx264 @ 000001a481da7080] mb I  I16..4: 34.4% 48.6% 17.1%
[libx264 @ 000001a481da7080] mb P  I16..4:  1.5%  4.4%  0.9%  P16..4: 32.5% 14.7%  9.3%  0.0%  0.0%    skip:36.8%
[libx264 @ 000001a481da7080] mb B  I16..4:  0.1%  0.1%  0.0%  B16..8: 18.5%  2.0%  0.4%  direct: 1.3%  skip:77.6%  L0:46.9% L1:42.6% BI:10.5%
[libx264 @ 000001a481da7080] 8x8 transform intra:60.5% inter:61.4%
[libx264 @ 000001a481da7080] coded y,uvDC,uvAC intra: 40.1% 47.4% 14.3% inter: 11.4% 6.1% 0.6%
[libx264 @ 000001a481da7080] i16 v,h,dc,p: 52% 15%  8% 24%
[libx264 @ 000001a481da7080] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 19% 16% 36%  3%  5%  5%  6%  5%  4%
[libx264 @ 000001a481da7080] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 28% 21% 14%  4%  7%  7%  9%  5%  5%
[libx264 @ 000001a481da7080] i8c dc,h,v,p: 66% 11% 15%  8%
[libx264 @ 000001a481da7080] Weighted P-Frames: Y:25.8% UV:12.1%
[libx264 @ 000001a481da7080] ref P L0: 59.9% 23.7% 14.8%  1.5%
[libx264 @ 000001a481da7080] ref B L0: 69.1% 19.4% 11.5%
[libx264 @ 000001a481da7080] ref B L1: 94.4%  5.6%
[libx264 @ 000001a481da7080] kb/s:204.50