Broken roundtrip in case of year<1000

[notEvil]

Hi,

if the year of a date is <1000 then the resulting invoice is invalid because %Y doesn't add 0 padding on all platforms, %4Y does.

python-drafthorse/drafthorse/models/elements.py

Line 337 in 536ca73

node.text = self._value.strftime("%Y%m%d")

Those dates are obviously wrong, however we do adjust invoices only in certain places and would like to keep the rest as it was.