Periodical cover date refers to the date displayed on the covers of magazines. The standard practice is to display on magazine covers a date which is some weeks or months in the future from the actual publishing/release date. The reason for this apparent discrepancy is to inform newsstands or Newsagents when an unsold magazine can be removed from the stands and returned to the publisher or be destroyed.
Weeklies (such as Time (magazine) and Newsweek) are generally dated a week ahead.
Monthlies (such as National Geographic Magazine) are generally dated a month ahead.
Quarterlies are generally dated three months ahead.
Comic books
The general practice of most mainstream comic book companies since the creation of the comic book in the 1930s was to date individual issues putting the name of a month (and much later the year as well) on the cover which was generally two months after the actual release date. For example, a 1951 issue of Superman which had the cover date of July would have been published two months earlier from that date in the month of May, generally speaking. In 1973 the discrepancy between the cover date and the publishing date went from two months to three months. In 1989 the cover date and publishing date discrepancy was changed back to two months, though generally each comic book company now uses its own system.