In general you receive your first Schengen-visa with a validity covering only the duration of your planned stay in the Schengen-Area (e.g. from November 1 to 12).
For subsequent travel(s), the Embassy can issue Schengen-visa(s) that permit the holder to stay in the Schengen-Area for a certain number of days within a certain period of time (e.g. 30 days between March 1 and May 31).
The maximum duration of stay in the Schengen-Area is always the number of days that are stated in the visa! The duration is counted from the date of the first entry into the Schengen-Area.
A Schengen-Visa with the validity of one year or more and a registered duration of stay of 90 days permits you, however, to stay in the Schengen-Area for a maximum period of 90 days per half-year (in Germany or other Schengen-Countries). Please note that the counting of the duration of stay does not begin with the start of the validity of the visa.
The calculation for the permissible duration of stay in the Schengen-Area was changed for travellers from non-European countries who require a Schengen-Visa to enter, as well as for those travellers for whom the visa requirement is waived. The change is effective as of 18 October 2013 and is made in accordance with Article 5, paragraph 1 of the Schengen Border Code. The previous “view ahead” (i.e. “90 days within 6 months”, i.e. counting forwards from the date of entry), is no longer applicable. Instead, a “flexible rearward view” is now applied (I.e. counting backwards from the “date of the entry/control”).
What does this new regulation mean for the traveller?
A person has a Schengen-Visa that is valid from 1 January 2013, until 31 December 2013, i.e. for one year.
On 18 October 2013, this person (i.e. the visa holder) is in the Schengen-Area.
In order to calculate the legitimacy of this visa holder’s stay in the Schengen-Area on this date (October 18, 2013), the period of time from 22 April 2013 until 18 October 2013 is examined. That is exactly the period of 180 calendar days, which ends on 18 October 2013.
Then within this period of time (the 180 calendar days) all those days are counted, in which the visa holder has been in the Schengen-area, or has entered or exited from it.
If the number of these days is not more than 90, then the visa holder’s stay in the Schengen-Area is legitimate on 18 October 2013.
If the visa holder does not exit the Schengen-Area, then the legitimacy of his stay is calculated again on the following day. In that case the period of time (180 calendar days) is moved by one day, and thereby changing its start- and end dates so it looks as follows: 23 April 2013, until 19 October2013.
This type of calculation is carried out for each day in which the visa holder is in the Schengen-Area.
You can access the calculator for the remaining travel days through the following link: http://ec.europa.eu/dgs/home-affairs/what-we-do/policies/borders-and-visas/border-crossing/schengen_calculator_en.html?lang=en