Upvote:5
Schengen visa have two relevant entries, duration and validity.
So you must arrive on or after the first day of validity, you must leave on or before the last day of validity, and you may stay no more than the duration. You must meet all these requirements.
It is common to issue Schengen visas with a few extra days or even weeks of validity, so you can adjust your flights without getting a new visa, but this does not always happen. This depends on the number of previous visits without overstay or the like, and on how solid your reasons to return appear to the visa officials.