The following 3 versions were analyzed via FCEUX 2.2.3:

NTSC-U PRG0 NTSC-U PRG1 (alternatively, "Revision A") NTSC-J

For reference, the NES NTSC framerate = 60.098813897441

In all 3 versions, the first frame of input to the character select screen appearing takes exactly 12 frames, or roughly 0.19967 seconds. With that said, if you record your runs at 60 FPS then feel free to find the first frame in which the "Please Select Player" screen appears, and then subtract frames accordingly. Lower framerates should be avoided if possible (for example, if you record at 30 FPS then there's a chance the first frame of character select would have been the second frame at 60 FPS, thus subtracting the amount of time detailed above results in a frame erroneously saved).

PRG1 and NTSC-J have slower loads between stages involving either doors or eagle heads. Exactly 1 frame is lost compared to PRG0 in each of these instances. Eagle heads that lead to different rooms within stages have no difference (e.g. 1-3 eagle head to Mouser), and the same is true for doors. No differences seem to exist for warp vases.

For end timing, I suggest we simply use the first frame the screen turns solid blue upon entering the final door.

Images for reference: https://imgur.com/a/t2nsc

Right now for competitive/close runs, the following timing method is used:

"Times that are very close to first will be timed from first frame after the loading text disappears to first frame of final spellcard bonus text (removes loading and is more precise"

The loading text fades away in an awkward manner to the point where it blends in with the background, thus making it difficult to pinpoint the exact frame it disappears in full. I'm suggesting time start simply on the frame the player character appears.

New start: Same End:

So as of this moment, "All Levels" as a category name is a bit of a misnomer. The primary issue is allowing for Mouser skip: a trick that involves a forced game over in 1-2 and then warping from 1-1 to 1-3, thereby circumventing the need to formally complete 1-2. One can argue--through loose interpretation--that entering 1-2 meets the requirements set by "All Levels", but I'm in the camp that treats this name as completion rather than mere visits. If you're unsure about whether or not 1-2 is actually flagged as completed in the game code, defeating Wart with Mouser skip results in a final tally of 19 levels during the podium sequence at the end (there are 20 levels in SMB2).

Now, what I'm suggesting isn't necessarily the banning of a trick. I understand that a lot of runs on the leaderboard employ this route and I would prefer not to obsolete times wholesale. I would argue that the easiest solution here, if one is taken, is to rename the category to something like "All Worlds" or whatever else is deemed most appropriate.

Ultimately I'm leaving this up for discussion, so please let me know what you guys think.

Since there seems to be renewed interest in this game, I thought it'd be meaningful to make a post about this glitch. Basically it's possible to clip through the floor (in particular, at the intersection of two lines). I'm not quite sure how this works, but I imagine a number of movement variables come into play:

Fortunately in this stage, there's actually floor immediately below the corner you can clip through.