Aside from a multiple-entry visa, which enables you to remain in the Schengen area for a maximum of 90 days within a 180 days period, the validity of your single-entry visa or double-entry visa depends on the embassy. The embassy appoints the number of days you are permitted to stay in any of the Schengen countries, as well as the first date you are permitted to enter Schengen and the last day that you are permitted to remain.

This is the point where many people get confused because they do not realize there is a difference between visa validity and the duration of stay in a visa.

Duration of Stay – the maximum of days you are permitted to remain in the Schengen. The first day you enter Schengen is counted as “Day 1”, even if you enter just a few minutes before midnight. Whereas, the “Last Day” is counted as the day you leave Schengen, even if it is just a few minutes after midnight.

Visa Validity – on the other hand, is the period of time from which to which you can use your visa to enter and stay in the Schengen Area.

For example, the duration of stay on your visa is 10 days, whereas the validity of your visa is from 1 January to 20 January. In this case, you can enter the Schengen Zone anytime within this period. You can enter i.e. on 3 January and leave on 12 January.  On the other hand, if you enter on 15 January, you will still have to leave on 20 January, despite of not having spent the number of days you were permitted to stay.

In the other case, if you have a double-entry visa, with a visa valid from January 1 to March 1, and duration of stay of 10 days, then you can enter the Schengen Area twice within this period. You are still not allowed to remain for more than 10 days during both trips. If during the first trip you stay 7 days, then on the second trip you can stay for three days at most. If you make just one trip and spend 10 days in any of the member states, then you have no right to enter the country despite the fact that your visa is still valid.

As per a multiple-entry visa, if i.e. you have a three-year multiple-entry visa, which becomes valid on January 15, 2018, then you will be permitted to enter and leave the Schengen whenever you want until January 15, 2021. You should not forget that there is a rule of 90 days limit per each 180-day period.