Support for (some) EDTF Level 1 and 2

[wolfsgrove]

EDTF Level 1 and 2 have several good candidates for inclusion, mostly those with hard boundaries. Some examples:

• Sub-year groupings (Quarters 1, 2, 3, 4)
• Set presentation (all included)
• Y -notation for years with more than 4 digits

Soft boundaries could visually be represented with transparency at the ends of the period in question to enable other Level 1 and 2 periods.