Quoting Alexander Strasser (2021-05-15 20:20:30)
> Hi Anton!
>
> On 2021-05-14 10:09 +0200, Anton Khirnov wrote:
> > Quoting Alexander Strasser (2021-05-12 01:04:28)
> > >
> > > If the timezone data of an AMF 0 date type is non-zero, include that
> > > information as UTC offset in hours and minutes.
> > > ---
> > > libavformat/flvdec.c | 18 +++++++++++++++---
> > > 1 file changed, 15 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
> > > index ddaceaafe4..c941e62e23 100644
> > > --- a/libavformat/flvdec.c
> > > +++ b/libavformat/flvdec.c
> > > @@ -688,9 +688,21 @@ static int amf_parse_object(AVFormatContext *s,
> > > AVStream *astream,
> > > time = date.milliseconds / 1000; // to seconds
> > > gmtime_r(&time, &t);
> > >
> > > - // timezone is ignored, since there is no easy way to offset
> > > the UTC
> > > - // timestamp into the specified timezone
> > > - strftime(datestr, sizeof(datestr), "%Y-%m-%dT%H:%M:%SZ", &t);
> > > + strftime(datestr, sizeof(datestr), "%Y-%m-%dT%H:%M:%S", &t);
> > > +
> > > + if (date.timezone) {
> > > + int off_tz = date.timezone; // offset in minutes
> > > + char ch_sign = '+';
> > > + if (off_tz < 0) {
> > > + off_tz = -off_tz;
> > > + ch_sign = '-';
> > > + }
> > > + if (off_tz > 99*60 + 59)
> > > + off_tz = 99*60 + 59;
> > > +
> > > + av_strlcatf(datestr, sizeof(datestr), "%c%02d%02d",
> > > ch_sign, off_tz / 60, off_tz % 60);
> >
> > I still believe this is wrong, since the spec says the timestamp is in
> > UTC. The code you quoted seems to conform to that.
>
> Not to my understanding and testing.
>
> This Ruby program
>
> t1 = Time.now()
> t2 = Time.now.utc()
> print t1, " - ", t1.to_f, "\n"
> print t2, " - ", t2.to_f, "\n"
>
> yields for example:
>
> 2021-05-15 20:05:19 +0200 - 1621101919.509961
> 2021-05-15 18:05:19 UTC - 1621101919.509966
This is just proving my point: the timestamp is UTC in both cases. The
only difference is that one of them has a non-zero timezone offset
attached.
>
>
> Returning to the code I quoted before now and stating my
> understanding of if now.
>
> def write__AMF_date(time)
> write__UI8 11
> write [(time.to_f * 1000.0)].pack('G')
> write__SI16( (Time.now.gmtoff / 60).to_i )
> end
>
> This writes the time in micro seconds without offset as double.
> The GMT offset in minutes is written afterwards as signed 16 bit
> integer.
>
>
> def read__AMF_date
> utc_time = Time.at((read__AMF_double / 1000).to_i)
> utc_time + (read__SI16 * 60) - Time.now.gmtoff
> end
>
> This first reads the double and converts it into a Time object.
> In the following line it reads and adds the stored offset and
> subtracts the current offset to get rid of its influence.
The writing code looks correct, but the reading code seems suspicious.
To test it, I used flvtool2 to write metadata with TZ=America/New_York
(-04:00) at 19:08 UTC. Then reading it with TZ=Europe/Paris (+02:00)
gives me:
- ffmpeg 4.3.2 -- correct, prints date in local timezone
metadatadate : Sun, 16 May 2021 21:08:41 +0200
- current ffmpeg git master -- correct, prints date in UTC
metadatadate : 2021-05-16T19:08:41Z
- flvtool2 -- WRONG
metadatadate: Sun May 16 15:08:41 GMT+0200 2021
>From which I conclude that flvtool2's own reading code is incorrect.
--
Anton Khirnov
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