[setr]

According to this https://www.learningtypescript.com/articles/why-typescript-d...

The versioning logic is fairly simple (although kind of pointless afaict?). X.1 —> X.9, and instead of going to .10 it simply increments major number. With average releases every 3 months, then major versions are simply being bumped roughly every 30 months.

I think functionally it’s just a really awkward date-versioning on a 30 month calendar instead of 12

[int_19h]

It's an old school versioning system that was very popular for DOS software. I'm not surprised that TypeScript, being ultimately still an Anders Hejlsberg project, would adopt it.