Schengen Area definition of 180 day period

[erfe]

is the 180 day period from the day of first entry through to 180 days later; or is it a constantly "moving" period (i.e. only 90 days of ANY 180 day period can be spent in the Schengen Area)? Note: this is for an Australian passport holder

Is this itinerary legal:
Day 1-30: Schengen area
31-120: Non- Schengen area
121-180: Schengen area (end of 180 day period after first entry, 90 days in Schengen area)
181-190: Non- Schengen area
191-280: Schengen area

By the end of the itineray the Australian passport holder has spent 150 of the previous 180 days in the Schengen Area; although potentially across 2 different "cycles" (days 1-180 and 191-370)

[danisara]

Hello, article 1 of the chapter 3 in the title 2 (definition of short-term visa) of the Schengen Acquis reads the following:

Code:

"(a) a travel visa valid for one or more entries, 
provided that neither the length of a continuous visit 
nor the total length of successive visits exceeds three 
months in any half-year, from the date of first entry;"

Hope that helps.