Unspecified parts of dates

[rybesh]

EDTF Levels 1 and 2 allow for parts of dates to be left unspecified. In Level 1 only smaller parts can be left unspecified; for example if the year is specified the day and month can be left unspecified, but if the month is specified the year cannot be left unspecified. In Level 2 any part can be left unspecified without restrictions.

This is easily modeled using the Time Ontology; for example to express the fact that the day and month are left unspecified we simply assert that a datetime description has day precision, but omit any triples specifying the month or day:

:when a time:Instant ;
  # Year specified, day and month unspecified
  # in a year-month-day expression  (day precision).
  edtfo:inEDTFDateTime "1985-XX-XX" ;
  time:inDateTime  [
    a time:DateTimeDescription ;
    time:unitType time:unitDay ;
    time:year 1985
    ]
  .

More examples:

• level-1/month-unspecified.ttl
• level-1/day-unspecified.ttl
• level-2/unspecified-digit-3.ttl

One complication is that EDTF also allows the rightmost digits of years to be left unspecified, e.g. 198X or 20XX. In order to handle these cases, we need to be able to specify parts of datetime descriptions with units larger than a year. For example, 198X refers to a year sometime in the 1980s, but we don’t know which year. We’d like to be able to model this with a datetime description that has year precision (since it is referring to a specific year) but which specifies the decade (1980s) without specifying the year.

For cases like this the draft EDTF Ontology defines three properties for specifying the parts of a datetime description at granularities larger than a year:

• decade
• century
• millennium

These take integer values such as 197 to refer to the 1970s, or 19 to refer to the 20th century. For example:

:when a time:Instant ;
  # A year with one rightmost unspecified digit
  # in a year-only expression (year precision).
  edtfo:inEDTFDateTime "201X" ;
  time:inDateTime  [
    a edtfo:DecadeDescription ;
    time:unitType time:unitYear ;
    edtfo:decade 201
    ]
  .

More examples:

• level-1/two-rightmost-unspecified-digits.ttl
• level-2/unspecified-digit-1.ttl
• level-2/unspecified-digit-2.ttl
• level-2/unspecified-digit-4.ttl
• level-2/unspecified-digit-5.ttl

Finally, just to make things extra complicated EDTF Level 2 also allows for digits within a month to be left unspecified; for example 1X means October, November, or December, and presumably X1 means January or November. These cases differ from the ones above in that here we cannot capture the unspecified parts of the date by creating a description with some parts left out: January and November are not parts of some larger unit we might identify.

So, the only way I see to handle these cases is to treat them as analogous to EDTF “one of a set” expressions (see discussion #5). See level-2/unspecified-digit-6.ttl for an example.