Incorrect value of #EXT-X-PROGRAM-DATE-TIME added in absence of the flag

[Yash]

Summary of the bug:
#EXT-X-PROGRAM-DATE-TIME tag gets added in parsing child playlist even if the child playlist is not created with program_date_time flag while using append_list HLS flag.

How to reproduce:
First command

% ffmpeg -loglevel debug -i BigBuckBunny.mp4 -ss 0 -t 30 -hls_list_size 0 -hls_playlist_type event -hls_flags independent_segments -hls_segment_type fmp4 -hls_time 2 -master_pl_name master.m3u8 -map 0:v:0 -map 0:a:0 -c:v copy -c:a copy -var_stream_map "v:0,a:0,name:1080p" -f hls %v.m3u8
ffmpeg version 4.4.git
built on ...
Ubuntu 20.04 5.11.0-38-generic

Second command

% ffmpeg -loglevel debug -i BigBuckBunny.mp4 -ss 0 -t 30 -hls_list_size 0 -hls_playlist_type event -hls_flags independent_segments+append_list -hls_segment_type fmp4 -hls_time 2 -master_pl_name master.m3u8 -map 0:v:0 -map 0:a:0 -c:v copy -c:a copy -var_stream_map "v:0,a:0,name:1080p" -f hls %v.m3u8
ffmpeg version 4.4.git
built on ...
Ubuntu 20.04 5.11.0-38-generic

Output child playlist after the first command

#EXTM3U
#EXT-X-VERSION:7
#EXT-X-TARGETDURATION:2
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-PLAYLIST-TYPE:EVENT
#EXT-X-INDEPENDENT-SEGMENTS
#EXT-X-MAP:URI="init.mp4"
#EXTINF:2.500000,
1080p0.m4s
#EXTINF:2.500000,
1080p1.m4s
#EXTINF:2.500000,
1080p2.m4s
#EXTINF:2.500000,
1080p3.m4s
#EXTINF:1.916667,
1080p4.m4s
#EXTINF:2.500000,
1080p5.m4s
#EXTINF:1.375000,
1080p6.m4s
#EXTINF:2.500000,
1080p7.m4s
#EXTINF:2.500000,
1080p8.m4s
#EXTINF:2.291667,
1080p9.m4s
#EXTINF:2.500000,
1080p10.m4s
#EXTINF:2.500000,
1080p11.m4s
#EXTINF:1.916667,
1080p12.m4s
#EXT-X-ENDLIST

Output child playlist after the second command

#EXTM3U
#EXT-X-VERSION:7
#EXT-X-TARGETDURATION:2
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-PLAYLIST-TYPE:EVENT
#EXT-X-INDEPENDENT-SEGMENTS
#EXT-X-MAP:URI="init.mp4"
#EXTINF:2.500000,
1080p0.m4s
#EXTINF:2.500000,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:02.500+0530
1080p1.m4s
#EXTINF:2.500000,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:05.000+0530
1080p2.m4s
#EXTINF:2.500000,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:07.500+0530
1080p3.m4s
#EXTINF:1.916667,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:10.000+0530
1080p4.m4s
#EXTINF:2.500000,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:11.917+0530
1080p5.m4s
#EXTINF:1.375000,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:14.417+0530
1080p6.m4s
#EXTINF:2.500000,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:15.792+0530
1080p7.m4s
#EXTINF:2.500000,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:18.292+0530
1080p8.m4s
#EXTINF:2.291667,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:20.792+0530
1080p9.m4s
#EXTINF:2.500000,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:23.083+0530
1080p10.m4s
#EXTINF:2.500000,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:25.583+0530
1080p11.m4s
#EXTINF:1.916667,
#EXT-X-PROGRAM-DATE-TIME:1970-01-01T05:30:28.083+0530
1080p12.m4s
#EXT-X-DISCONTINUITY
#EXTINF:2.500000,
1080p13.m4s
#EXTINF:2.500000,
1080p14.m4s
#EXTINF:2.500000,
1080p15.m4s
#EXTINF:2.500000,
1080p16.m4s
#EXTINF:1.916667,
1080p17.m4s
#EXTINF:2.500000,
1080p18.m4s
#EXTINF:1.375000,
1080p19.m4s
#EXTINF:2.500000,
1080p20.m4s
#EXTINF:2.500000,
1080p21.m4s
#EXTINF:2.291667,
1080p22.m4s
#EXTINF:2.500000,
1080p23.m4s
#EXTINF:2.500000,
1080p24.m4s
#EXTINF:1.916667,
1080p25.m4s
#EXT-X-ENDLIST

Video:​http://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4

[Yash]

first_command_git_head_debug_log

[Yash]

second_command_git_head_debug_log

[Yash]

child_playlist

[Yash]

Seems to have been introduced while fixing ​https://trac.ffmpeg.org/ticket/8989. Following changes needed at ​https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/hlsenc.c#L1290

From

vs->last_segment->discont_program_date_time = discont_program_date_time;
discont_program_date_time += vs->duration;

To

if (discont_program_date_time > 0) {
    vs->last_segment->discont_program_date_time = discont_program_date_time;
    discont_program_date_time += vs->duration;
}