The Medical Subject Headings (MeSH) in PubMed is the National Library of Medicine’s controlled vocabulary used for indexing articles. These terms are helpful in retrieving publications on a specific topic. Some MeSH records will include a year to indicate when the subject heading was introduced to the database. But often times, the MeSH record may provide two distinct years, while other records may provide none. To learn more about dates in MeSH, see this recent video (1:48 min) from the National Library of Medicine Training Center.