Intl.RelativeTimeFormat helps libraries and frameworks format relative time in a localized fashion by providing internationalized messages for date and time fields, using a customary local word or phrase when available. For example, calling its format() method with -5 and 'day' would return the string '5 days ago'. Intl.RelativeTimeFormat is available behind a flag in Firefox 58.
Docs: https://developers.google.com/web/updates/2018/10/intl-relativetimeformat