I haven't done this myself. I'm just the messenger passing along info
I came across two pages that seem to imply that you'll need to create a second video (presumably the time display) and then overlay it on the original video. Again, I haven't done it myself, but it seems like a reasonable approach and I did see one or two pages where someone said, flat-out, that ffmpeg cannot burn a time onto a video itself.
The pages I found:
ffmpeg overlay time offset
simple overlay and logo