The time code is always incrementing by one frame at the given frame rate. For any of the NTSC-derived framerates, there are then two ways of incrementing. You can increment as if the frame rate is integral--so after 30 frames at 29.97 fps, your timestamp will show that 1 second has passed. The other option is a "drop-frame" timecode, where you skip over certain numbers when incrementing.

In all cases, the time code increments at the frame rate you are using.