There's a lot of better calendars out there but the Cosworth is arguably the best. The additional month is called Sol and is between June and July. Additional days for leap years and last day of year are treated like free days and the calendar pauses for them.

Kodak used it and everything from finance to pay to reporting was a breeze. I get to use it and the 4–4–5 calendar often in data analysis so that historical data is easier to segment and compare.

Apparently it's better known as https://en.wikipedia.org/wiki/International_Fixed_Calendar