[FFmpeg-user] How is the gmtime option used for pts?

[Matthew Mah]

How is the gmtime option used for pts? I want to overlay the time video 
frames have been captured onto a video and display the time in a 
YYYY-MM-DD HH:MM:SS format. I have succeeded in using the pts option 
with flt to display the time in seconds since the start of the video.

ffmpeg -i in.webm -filter_complex 
"drawtext=fontfile=/usr/share/fonts/truetype/arial.ttf: text='%{pts 
\:flt}': x=100 : y=50 : box=1" -c:a copy out.webm

There is a gmtime option for pts described in the documentation:
> pts
> The timestamp of the current frame. It can take up to three arguments.
>
> The first argument is the format of the timestamp; it defaults to flt 
> for seconds as a decimal number with microsecond accuracy; hms stands 
> for a formatted [-]HH:MM:SS.mmm timestamp with millisecond accuracy. 
> gmtime stands for the timestamp of the frame formatted as UTC time; 
> localtime stands for the timestamp of the frame formatted as local 
> time zone time.
>
> The second argument is an offset added to the timestamp.
This gmtime option does not work for me, when running:
ffmpeg -i in.webm -filter_complex 
"drawtext=fontfile=/usr/share/fonts/truetype/arial.ttf: text='%{pts 
\:gmtime}': x=100 : y=50 : box=1" -c:a copy out.webm

I get the error: "Invalid format 'gmtime'".

How can the gmtime option be applied to display the time of the video in 
YYYY-MM-DD HH:MM:SS format?

I posted this question weeks ago to superuser.com, but did not get a 
response on using the pts gmtime argument:
http://superuser.com/q/1013753/483849