My issue is kinda similar to this one so I'm posting here: The Trim function is not working for me (probably because of a Bungee server switching) and synchronizing the timelines does nothing when I start rendering.
I don't see any way not to render from the very beginning of the scene but I'd like to know why the rendering is not made so it starts from the first time keyframe, I think it would make sense.
Time keyframes don't work that way. Rendering starts at 0:00 of the bottom timeline. The bottom timeline corresponds 1:1 to the resulting video.
Therefore if you place one time keyframe at 0:00 on the bottom timeline, it'll define the time in the replay at the beginning of the video. If you do not place it at 0:00 but e.g. at 0:15 it'll first render 15 seconds worth of video before reaching the time you want it to be at.