Okay. It was bugging me not to fully compare this to the spec, so I just went and looked again.

I'm obviously not well enough versed on how %s works with modifiers, since I didn't know that you could even put modifiers on %s. But assuming that %.4s is equivalent to %04d, and %.2s is equivalent to %02d, I'd say that this looks good. And it manages fewer `format` calls than my implementation (though my implementation is more flexible in terms of its output, complicating it a bit, and I haven't really tried to optimize it yet - correctness was far more critical, though I'll have to optimize it at some point).