From Google Scholar:
"Scholar Metrics currently cover articles published between 2010 and 2014, both inclusive. The metrics are based on citations from all articles that were indexed in Google Scholar in June 2015. This also includes citations from articles that are not themselves covered by Scholar Metrics.
Since Google Scholar indexes articles from a large number of websites, we can't always tell in which journal a particular article has been published. To avoid misidentification of publications, we have included only the following items:
Furthermore, we have specifically excluded the following items:
Overall, Scholar Metrics cover a substantial fraction of scholarly articles published in the last five years. However, they don't currently cover a large number of articles from smaller publications."
Google Scholar has adapted the h-index method of impact for publications and an h5 variation for five complete calendar years. From Google Scholar:
In publishing the h5 index for their Top 100 publications in Google Scholar Metrics, the top articles cited are also included. When viewing the Top 100 publications, click on the h5 index score to see the most highly cited articles from that journal (h5-core). Then, you can click on the citation count for any article in the h5-core to see who cited it.
This disclaimer accompanies the top articles cited: "Dates and citation counts are estimated and are determined automatically by a computer program."