The result of Builder#build. Holds a list of Year objects and adds a few extra methods.
an array of all the Days of all the Months. If proxies=false (the default) strip Days before starting and after ending
Day of first day of first month of first year that is or isn‘t a proxy
Day of last day of last month of last week that is or isn‘t a proxy
an array of all the Months of all the Years
[Validate]