A lot of languages take their date/time formatting from C strftime (and quite a few simply use light wrappers around actual strftime), where the format code for ISO year is %G.
And FWIW, Python's (strftime-based) datetime library won't let you mix ISO and non-ISO format codes. Trying to use %G with %m, for example, raises an exception, as does trying to use %Y with %V (%V is the ISO week number format code).
And FWIW, Python's (strftime-based) datetime library won't let you mix ISO and non-ISO format codes. Trying to use %G with %m, for example, raises an exception, as does trying to use %Y with %V (%V is the ISO week number format code).