Short stay visa technicalities, 6 months or 180 days

[4000ft]

My gf has a Type C visa valid for 2 years, both the european aquis and the visa code (810/2009) use months to define the allowable stay and total time periods, and use 90 days interchangeably with 3 months. The problem is nowhere in the documents are 180 days referenced, it is always half-year or 6 months. Also the european convention on time limits defines a month as being from the day in a month to the same day in the next month, or if the day does not exist it is until the last day of the month (eg. 31st jan to 28 feb).

Her first entry (30/07, for 2 weeks, then she left for 2 months) on the visa will be 180 days ago this coming friday and her cumulative stays will be 90 also, so if she stays after friday she will be on exactly 90 days for the next 2 weeks. However if going by six months she will have to leave friday and return on or after the 31st.

Can anyone provide an official source for the 180 days rule?, something that is clear in saying 180 days rather than six months or half year...

Also, we've asked a number of consulate employees and officials at the airport on her last trip out and about 50% of them are convinced her total stay is only 90 days over 2 years, is that a possibility or just nonsense?.