These are a lot like TimelineDayRow and TimelineDayModel, and are
supposed to go in the timeline’s header too, but instead of showing the
day number, they show the month name.
I still need to know the with of each day, because the month must be as
wide as all shown days, that may not be all the days in that month, in
the first and last months, at least.
Usually i will show two or three months, so i thing is is fair to keep a
list of all the months and their days, rather than compute them each
time the name or the day count, two times per month, are requested.