# Intl Utils
Provide i18n utilities.
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## API
### selectUnit
This function determines the `best fit` unit based on a specific set of customizable thresholds.
```ts
function selectUnit(
from: Date | number,
to: Date | number = Date.now(),
thresholds = DEFAULT_THRESHOLDS
): {value: number; unit: Unit};
```
where `thresholds` has the shape of:
```ts
interface Threshold {
second: number;
minute: number;
hour: number;
day: number;
}
```
`month` & `year` are based on calendar, thus not customizable.
Example:
```ts
import {selectUnit} from '@formatjs/intl-utils';
selectUnit(Date.now() - 1000); // { value: -1, unit: 'second' }
selectUnit(Date.now() - 44000); // { value: -44, unit: 'second' }
selectUnit(Date.now() - 50000); // { value: 1, unit: 'minute' }
```
#### Caveats
`selectUnit` is meant to be a stepping stone from the old `IntlRelativeFormat` to the officially spec-ed `Intl.RelativeTimeFormat`. Therefore we don't recommend using this for an extended period of time because of ambiguous editorial issues such as:
- From 2019/01/01 -> 2018/11/01 can technically be `last year`, `2 months ago` or `a quarter ago`.
- From 2019/01/02 6am to 2019/01/01 11pm can also be `7 hours ago` or `yesterday`. Timezone further complicates the issue.
The examples above have not even tackled the differences in non-Gregorian calendars. There is an issue opened upstream in the spec that potentially introduces a [`best fit` algorithm](https://github.com/tc39/proposal-intl-relative-time/issues/47). Therefore, we recommend that you implement your own version of `selectUnit` that matches your editorial expectation.