The starting point to use when variable-length units (years, months, weeks depending on the calendar) are involved. This option is required if either of the durations has a nonzero value for `weeks` or larger units. This value must be either a `Temporal.PlainDateTime`, a `Temporal.ZonedDateTime`, or a string or object value that can be passed to `from()` of those types. Examples: - `'2020-01-01T00:00-08:00[America/Los_Angeles]'` - `'2020-01-01'` - `Temporal.PlainDate.from('2020-01-01')` `Temporal.ZonedDateTime` will be tried first because it's more specific, with `Temporal.PlainDateTime` as a fallback. If the value resolves to a `Temporal.ZonedDateTime`, then operation will adjust for DST and other time zone transitions. Otherwise (including if this option is omitted), then the operation will ignore time zone transitions and all days will be assumed to be 24 hours long.