Dates are used in numerous places in the OrganSpec DTD, all of which must conform to the following syntax:

['c'] yyyy ['-' mm ['-' dd]]

• The year is always required, and must be four digits long (e.g. "1985", not "85").
  
• The month is optional, must consist of 1 or 2 digits, and ranges from 1 to 12.
  
• The day is optional, must consist of 1 or 2 digits, and ranges from 1 to 31.
  
• The day, month and year are separated by hyphens.
  
• If the day is specified, the month must also be specified.
  
• An optional "c" may precede a lone year to indicate an approximate date (i.e. "circa").

Thus, the following formats are allowed:

yyyy-mm-dd
yyyy-mm
yyyy
cyyyy

"mm" represents a 1 or 2 digit month, where "1" = January
"dd" represents a 1 or 2 digit day of the month, starting with "1"
"yyyy" represents a 4-digit year
"c" represents the letter 'c' or 'C', for "circa", when only an approximate year is known

With the exception of the optional leading 'c', this format is intentionally compatible with the XML Schema date format.