Google Scholar has adapted the h-index method of impact for publications and an h5 variation for five complete calendar years. From Google Scholar:

• The h-index of a publication is the largest number h such that at least h articles in that publication were cited at least h times each. For example, a publication with five articles cited by, respectively, 17, 9, 6, 3, and 2, has the h-index of 3.
• The h-core of a publication is a set of top cited h articles from the publication. These are the articles that the h-index is based on. For example, the publication above has the h-core with three articles, those cited by 17, 9, and 6.
• The h-median of a publication be the median of the citation counts in its h-core. For example, the h-median of the publication above is 9. The h-median is a measure of the distribution of citations to the h-core articles.
• Finally, the h5-index, h5-core, and h5-median of a publication are, respectively, the h-index, h-core, and h-median of only those of its articles that were published in the last five complete calendar years.

From Google Scholar:

"Scholar Metrics currently cover articles published between 2015 and 2019, both inclusive. The metrics are based on citations from all articles that were indexed in Google Scholar in June 2020. 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:

• journal articles from websites that follow our inclusion guidelines;
• selected conference articles in Computer Science and Electrical Engineering;
• preprints from arXiv, SSRN, NBER and RePEC - for these sites, we compute metrics for individual collections, e.g., "arXiv Superconductivity (cond-mat.supr-con)" or "CEPR Discussion Papers".

Furthermore, we have specifically excluded the following items:

• court opinions, patents, books, and dissertations;
• publications with fewer than 100 articles published between 2015 and 2019;
• publications that received no citations to articles published between 2015 and 2019.

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."