A banknote with 8-digits serial number that when it's read represent a date. These piece can be valuable to a collector based on an special date (aniversary, birthday, etc). The date have the following formats:

• YYYYMMDD: 4 digits year, 2 digits month and 2 digits day. Example: "20110509"
  
• DDMMYYYY: 2 digits day, 2 digits month and 4 digits year. Example: "05092011"
  
• MMDDYYYY: 2 digits month, 2 digits day and 4 digits year. Example: "09052011"

Example of Birthday serial number (year-month-day):
Example of Birthday serial number (month-day-year):

(Photograph credits: Nestor Lugo) Example of Birthday serial number (day-moth-year):

(Photograph credits: Nestor Lugo)

A banknote with a serial number that contains pairs of same digits, for example "11223344".

Example of Doubles serial number:

(Photograph credits: Mercadolibre: billetes.antiguos. This piece also is Binary serie and Bookend serial number (4 digits))