Every real photo postcard has three dates associated with it: the date the image was captured with a camera and a negative created, the date the negative was printed out on photographic paper to create the postcard, and the date the card was mailed and postmarked. These three dates are not necessarily contemporary. An image negative captured in the 1910s could be used for decades as the Eastern Illustrating & Publishing Company of Belfast, Maine often did or a 19th century photograph negative could be reprinted as a postcard as Frank de Mars of Winsted, Connecticut did with many negatives acquired from local photographer King Sheldon. Further, mailing a postcard could occur anytime after a postcard was printed out. In fact, many examples exist of cards being mailed 50 years or more after the card being printed out. In practice, however, such discrepancies were not the norm; most cards were photographed, printed out, and mailed within relatively short periods of time. Generally, priority is given to the date when a negative was printed out as a real photo postcard. If an image is printed out on a postcard long after the negative was made, the card is considered a reprint or reproduction.
Dating an image of a photograph more precisely than to a few years period can be challenging. Technology and fashion evolved but unless a photograph depicts a well known event or is captioned with a date, a date range is generally assigned. In contrast, the date a negative was printed out on photographic postcard stock is usually more exact. The period of time when most real photo postcard backs were current has been worked out by researchers. Due to chemical deterioration, most prepared photographic paper could not be stored beyond a couple of years so printing of a photograph on long discontinued paper is unlikely. If postally used, providing a date when a postcard was mailed is even more straight forward since postmarks give an exact date. If a postmark date is not decipherable, examination of the type of stamp, the postal rate, and the style of postmark allow a date range to be assigned.