If you have something like "between 1846 and 1855", you can use the "before" and
"after" fields of the time value:

  time: "+00000001850-00-00T00:00:00Z",
  precision: 9,
  before: 4,
  after: 5

This means the "main" value is 1850, given as a year, with a lower bound four
years before and an upper bound 5 years after the "main" value (before and after
are given in the unit specified by the precision value). The "main" value is
what is going to be displayed per default; it will also be used for sorting
query results (once we have queries).