The numerical display in the timeline is displaying timecode.
The format of timecode is always Hours:Minutes:Seconds:Frames (HH;MM;SS;FF).
The last two digits are frames, not portions of a second. The last two digits should always stop up counting 1 number before the frame rate of the timeline. The next number will be the start of the next second, and the frame count will revert to :00.
Changing the frame rate of time line will have consequences. You normally would not want to have a different frame rate on the timeline than the frame rate of your source material, unless you a very specific reason to do so.
The frame is the smallest single unit of video. You can only edit video at the frame points. There is no such thing as, for example, a 1/2 frame of video.
If you need greater precision for editing audio, you can edit audio on the timeline at a higher temporal resolution than the video frame rate by clicking on the hamburger menu next to the Sequence name and from the drop down menu choosing Show Audio Time Units:

Now the timeline will display in thousandths of a second, and the you can edit the audio a sub frame precision - but the video will always only be editable at the whole frame level.
MtD