This is the second question you've posted about this matter but you've not mentioned your nationality.
Since this site is UK-based, the majority of people who post here are British citizens. Any citizen of an EU-member country has the right to travel to any other EU country without visa restriction so, if you're British, you don't need a visa for Greece (or for Finland).
The next most common group of visitors to this site are probably those from the US. US passport holders, however, don't need a visa to travel to Greece. In fact, the citizens of most countries, from which large numbers of tourists travel from, don't require a visa for entry into Greece. These include Australia, New Zealand, Japan and many other countries. See here for a full list:
http://gogreece.about.com/cs/practicalinfo/a/g reecevisareqs.htm
If you're sure that you do need a visa, then a visa issued by the Finnish embassy is definitely valid (for a stay of up to 90 days) for any Schengen country, including Finland. It's almost certain that the visa won't specify a specific date for entry into a Schengen-member state but the easiest way to check would seem to be phone the Finnish embassy (in whichever country you made your application).
Chris