example, by changing the date. Practical uses The postmark is often considered as an official confirmation that a cover (letter, packet, etc.) mailed item was mailed at a given location at a specific date. For example, the date of the postmark can be quite important. In the United States, the Internal Revenue Service will still consider income tax returns as filed on time though it receives them late if they are postmarked on time, and this date (with, perhaps, other proof of mailing), may have significance in the context of legal filings and proofs of service or of delivery (though in this case the date may viewed as "on time" if the date of the postmark is no more than one day after the date service is supposed to have been made). Postal voting ballots may be accepted in some places if postmarked by the date of the election, though other places require receipt by a certain deadline. Historically, postmarks, known as backstamps were also applied to the reverse side of a cover to confirm arrival at the post office on a specific date. Similar marks A postmark should not be confused with the killer which are lines, bars, etc. used to cancel a postage stamp. The killer acts as the cancellation, though the postmark can also serve this purpose. Neither should a postmark be confused with overprints generally, or pre-cancels (stamps that have been cancelled before the envelope or package to which they are affixed is submitted or deposited for acceptance into the mailstream, they most commonly have taken the form of a pre-printed city name on the stamp) specifically, which generally do not indicate a date. Flight cachets, more or less elaborate rubber-stamps on an envelope indicating on which flight (typically a first flight), a first flight cover has traveled via airmail, are in addition to the postmark and are not postmarks either. Clubs There are many clubs devoted to the hobby of collecting postmarks. One of those clubs is the Post Mark Collector's Club, founded in 1946 and based in the USA. Another is the British Postmark Society, founded in 1958. See also Indicia (philately) Postal marking Postal history References External links Anonymous Postmark Mailer British Postmark database World postmark primer: how to decipher dates Geographic North Pole Postmark A Mystery Postmark London 2012 – New postmark kicks off 2012 day in London (backing London's bid to