How can I add an input source to ffmpeg so I can add the current datetime/timestamp/ISO-time as a seperate subtitle stream? E.g. something like this:

ffmpeg -i video.mp4 -i audio.mp3 -i <YYYY-MM-DDTHH:MM:SS.ffffff> -map 0:v -map 1:a -map 2:s -c:v copy -c:a copy -c:s srt -f flv output.flv

The idea is that ffmpeg always takes the current datetime for the specific frame that ffmpeg encodes at that moment, and not the (static) datetime when the ffmpeg process was started.