Months names are all one off?

[Jorge_Stolfi]

The astrological calendars that divide each sign always into 30 parts are not referring to days, but to degrees of angle on the Ecliptic, synchronized with an equinox or solstice.   Diagrams drawn by astronomers may well use this convention.  In those diagrams, each division is a tad longer than one day, and the boundaries drift along the year, and from year to year.  

The Chinese "agricultural" calendar too used degrees instead of days, but with 24 sectors of 15 degrees, instead of 12 sectors of 30.