DateTime matchDate = new DateTime(2012, 2, 20);
var query = from file in files
where (new FileInfo(file)).LastWriteTime.Date == matchDate
select file;
matchFiles = query.ToArray<string>();
or
DateTime matchDate = new DateTime(2012, 2, 20);
foreach (string file in files)
{
if ((new FileInfo(file)).LastWriteTime.Date == matchDate)
{
matchFiles.Add(file);
}
}