Some tags exist that can be purely represented by metatags. For instance, short_playtime is equivalent (by wiki definition) to duration:<30, and long_playtime to duration:>=30. Currently, aliasing them with those definitions is not possible, and it would be convenient.

Could special case it, but if there are other tags like this, modifying the system to handle it might be easier.

this is what bots are for, because I don't think this could ever be possible.

Looking further, the duration metatag doesn't work properly with gif files (some have it properly, some don't have duration filled out at all, despite it being automatic). There are over 750 pages of gif without any duration information, and only 39 posts that have it. Something weird with the format? Could it be fixed?
Anyway, that means that this alias wouldn't be quite correct, even if it was possible.
Webm too? Strange.
Ok, it looks like duration: only works for mp4 files.

scth said:
Looking further, the duration metatag doesn't work properly with gif files (some have it properly, some don't have duration filled out at all, despite it being automatic). There are over 750 pages of gif without any duration information, and only 39 posts that have it. Something weird with the format? Could it be fixed?
Anyway, that means that this alias wouldn't be quite correct, even if it was possible.
Webm too? Strange.
Ok, it looks like duration: only works for mp4 files.

Yeah, it's a bug/missing feature in the parser. Technically, nothing keeps you from calculating this.