$ ffmpeg -i timer-flags.flv -c copy timer-flags.mp4
ffmpeg version 3.4 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 7.2.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-avisynth --enable-avresample --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libass --enable-libbluray --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxvid --enable-shared --enable-version3
  libavutil      55. 78.100 / 55. 78.100
  libavcodec     57.107.100 / 57.107.100
  libavformat    57. 83.100 / 57. 83.100
  libavdevice    57. 10.100 / 57. 10.100
  libavfilter     6.107.100 /  6.107.100
  libavresample   3.  7.  0 /  3.  7.  0
  libswscale      4.  8.100 /  4.  8.100
  libswresample   2.  9.100 /  2.  9.100
  libpostproc    54.  7.100 / 54.  7.100
Input #0, flv, from 'timer-flags.flv':
  Metadata:
    encoder         : Lavf57.83.100
  Duration: 00:00:39.93, start: 0.033000, bitrate: 2694 kb/s
    Stream #0:0: Video: h264 (High), yuv420p(progressive), 1280x720, 2500 kb/s, 62.50 fps, 60 tbr, 1k tbn, 120 tbc
    Stream #0:1: Audio: aac (LC), 44100 Hz, stereo, fltp, 160 kb/s
Output #0, mp4, to 'timer-flags.mp4':
  Metadata:
    encoder         : Lavf57.83.100
    Stream #0:0: Video: h264 (High) (avc1 / 0x31637661), yuv420p(progressive), 1280x720, q=2-31, 2500 kb/s, 62.50 fps, 60 tbr, 16k tbn, 1k tbc
    Stream #0:1: Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 160 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
Invalid return value 0 for stream protocol