EDTF and hence ISO/DIS 8601-2 section 4.10 specify that a date of exactly three digits and nothing else signifies a decade, i.e. CCY. This would align with ISO 8601:2004’s and ISO/DIS 8601-1’s 2-digit CC format. Alas, this is one of the least used and most useless and most confusing definitions of the standard!
Most people would intuitively associate a number consisting of just two digits as either a month MM or a day of a month DD, both part of the default, characteristic and ubiquitous CCYY-MM-DD (or CCYYMMDD) ISO date format. These could, of course, be unambiguously written -MM and --DD instead, except that -MM would clash with -CC, because hyphen and minus use the same character in ASCII. Three digits are also used for ordinal days of the year in CCYY-DDD, which could be written -DDD which again would clash with -CCY, i.e. decades before the epoch.
I therefore suggest to use a different and new syntax for centuries CC and decades CCY, and millenia C as well. I have no strong preference, but I would imagine one of these:
- C___, CC__, CCY_: 1___, 19__, 196_
- C###, CC##, CCY#: 1###, 19##, 196#
- C***, CC**, CCY*: 1***, 19**, 196*
- C???, CC??, CCY?: 1???, 19??, 196?
- C‘XXX’, CC‘XX’, CCY‘X’: 1XXX, 19XX, 196X
- ‘K’C, ‘H’CC, ‘D’CCY: K1, H19, D196 – from SI prefixes kilo, hecto, deca/deka/(deci)
- C‘K’, CC‘H’, CCY‘D’: 1K, 19H, 196D – from SI prefixes kilo, hecto, deca/deka/(deci)
- ‘M’C, ‘C’CC, ‘D’CCY: M1, C19, D196 – from Romance-English ‘millenium’, ‘century’, ‘decade’
- C‘M’, CC‘C’, CCY‘D’: 1M, 19C, 196D – from Romance-English ‘millenium’, ‘century’, ‘decade’