Relative-time format patterns are of one of two forms:
* Tensed - refer to the past or the future, e.g. "N years ago" or
"in N years".
* Numbered - refer to a specific numeric value, e.g. "in 1 year"
becomes "next year" and "in 0 years" becomes "this year".
In ECMA-402, tensed and numbered refer to the numeric formatting options
of "always" and "auto", respectively.
