ISO 8601-2 is a “draft international standard” (DIS) right now, on ballot through March. In section 4.5 it is specifying year-only dates which extend beyond 4 digits without the need for further conventions, as proposed by EDTF in <https://www.loc.gov/standards/datetime/pre-submission.html#yearexceedingfourdigitsl1> (level 1) and <https://www.loc.gov/standards/datetime/pre-submission.html#yearexceedingfourdigitsexponentialform> (level 2, exponential form). I believe this approach could be improved to be more useful by documenting a existing pattern. I’m planning to also submit this feedback through my national standards body (NSB).
I agree that approximate years with many trailing zeros are clumsy. However, the public seems to prefer to replace them by letters inspired by SI decimal prefixes, instead of using exponential E notation: “Y2K” is a well known alternative representation (in English) for the year 2000.
‘K’ conventionally stands for ‘kilo’, i.e. a multiple of 1000, a thousand. Likewise, ‘M’ for ‘mega’ is a multiplier for a million and ‘G’ for ‘giga’ multiplies by a billion (or milliard). Since the age of the universe is estimated around 14 billion years and the remaining lifespan is perhaps in a similar order of magnitude, larger multipliers – although defined in SI, e.g. ‘T’ for ‘tera’, trillion – are not useful for years.
The example used in EDTF/ISO, ‘y170000000’ = ‘Y17E7’, would therefore become ‘Y170M’ or, perhaps, ‘Y0,17G’, ‘Y.17G’ …
Also, the plus sign ‘+’ should be valid, though optional in positive years. (Actually, with little changes to the standard, a mandatory prefixed sign could replace the letter ‘Y’.)