Upvote:6
As initially pointed out by DavidSupportsMonica, you can enter the Schengen zone on the first day of validity. This applies as long as your changed itinerary is based on the itinerary from the application. Afterwards, you can use it for subsequent trips. But you need to avoid the perception that you lied in your original application to get the visa.
It is common to give a few extra days before and after the planned itinerary to allow minor adjustments of the itinerary. Just how "generous" they are depends on the credibility of your application, your circumstances, and probably also on the country which processes your application. Two years from the first of the month means they think you appear trustworthy.
So you should use your visa for something that has a recognizable relation to the trip you outlined in your application. You can change flights/airports and hence the country of entry, you can add a long weekend to get over jet lag and do some sightseeing on the side, meetings can get re-scheduled, etc. But if the visa was for a business trip to Iceland and you go to Greece for a vacation first, there may be questions.