Works poorly with negative-seeming years

[keysmashes]

For Reasons™, nixpkgs uses package versions like 0-unstable-2000-01-01 for packages without a version number. This doesn't work super well with vim-speeddating – trying to increment the day in that string, for example, produces the result 0-unstable0000-01-02.

Adding %?[-] in front of the built-in %Y-%m-%d format seems to help.