The bottom timeline is your video, the top one is the replay. When you put a time keyframe down on the bottom timeline at 1:00, you're effectively declaring that whatever the current time in the replay is, you want the time of the replay at 1 minute into the video to be the same.
So to make a simple timelapse, you skip to when you start building, put down the first time keyframe at 0:00 (the beginning of the video), then skip to the end (e.g. if you're making a 10x timelapse, skip 10 minutes ahead) and place your second time keyframe at 1:00.
If you're making a 6x (or any valid replay speed slider value) timelapse, you can also, instead of guessing the position of your next time keyframe on the bottom timeline, press V to automatically position the cursor where it belongs relative to the previous time keyframe according to your current replay speed slider setting.
Other than that, the slider is basically irrelevant to the bottom timeline, i.e. time keyframes overwrite it during playback.