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In 2004 the United States Postal Service announced plans to introduce first day digital colour postmarks to be used to cancel some first day covers for commemorative stamps in 2005 and this practice has continued into 2006 The study of postmarks is a specialized branch of philately called marcophily.
It may bring added value to the stamps by their historical significance.
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 proofs of mailing), may have significance as regards legal filings and proofs of service (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).
A postmark should not be confused with the killer which are lines, bars, etc. 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. national parks also have a postmark-like inked rubber stamp available to stamp into a book of park stamps (the Passport To Your National Parks) or a postcard, or any piece of paper.
This was used in conjunction with a date stamp which was applied, usually to the rear of the letter, which denoted the date of posting.
Similar to this is the "censored postmark," overprinted with a black obliteration of the time and place of mailing for similar reasons.While postmarks are applied almost universally by or under the authority of the official postal department, service, or authority in the United States it is possible to receive a permit to apply your own postmark, called a Mailer's Permit Postmark These include the so-called fancy cancels of United States to modern machine postmarks.Fewer postmarks are used now than previously, with the advent of meter labels, some types of computer vended postage, and computerized postage that people can print from their own PCs (called PC Postage in the United States, these services were offered by such companies as and Neopost, Inc.).More details can be found in Valuation of cancellations of the Austrian Empire.A special or rare postmark can substantially add to the value of a stamp.
A postmark is a postal marking made on a letter, package, postcard or the like indicating the date and time that the item was delivered into the care of the postal service.