EN 15907 Timespan
Definition from the standard
Elements of type Timespan shall contain representations of absolute periods of time with variable degrees of precision. Recommendations for a suitable syntax are given below.
Expression of time spans
Absolute time spans are based on the Gregorian calendar with a maximum precision of one calendar day. The notation uses the element order defined in ISO 8601 in the form YYYY-MM-DD (year, month, day).
Lower-precision dates may be expressed by setting the day, or day and month values to zero (e.g. "1950-08-00" for August, 1950 and "1950-00-00" for the year 1950). Decades may be represented by a question mark as the least significant digit of the year (e.g. "195?" for the decade 1950-1959), and centuries by two question marks (e.g. "19??" for the 20th century).
Time spans with known start and end dates may be represented by concatenating two date notations with as sequence of two dashes (Unicode value 002Dhex), e.g. "1979-12-15--1980-01-00" for a time span starting on the 15th of December, 1979 and extending to some date in January, 1980.
Relative time statements may be expressed by prefixing a calendar date, or range of dates, with one of the following:
before - any time before, but not including, the given date.
after - any time after, but not including, the given date.
between - any time between, but not including the two given dates.
started - any time from the given date until now.
ended - any time from the past up to, and including, the given date.
circa - the given date or any date shortly before or after that date. The given date can be regarded as the maximum of a probability function.