Check against test files

[badboy]

I stumbled upon https://ijmacd.github.io/rfc3339-iso8601/
I never actually read the iso8601 standard (for one because it's not freely available), so technically we're definitely not iso8601-compliant.
Might be worth looking into whether we parse data from the test file.

e.g. I'm pretty sure we don't correctly handle leading T like T10:47:21+01. Nor do we allow lowercase t.