Relative Time Formatter

A live workbench for the browser's built-in Intl.RelativeTimeFormat. Pick a target date and a base — by default now, ticking every second — and see the formatted string in the locale you choose. The unit table renders the same difference as seconds, minutes, hours, days, weeks, months, quarters and years at once, which is the fastest way to decide which granularity reads best in a given interface.

The numeric option is where the interesting behaviour lives. With always you get the plain arithmetic form: 1 day ago, in 1 day, and, at zero, the slightly awkward in 0 days. With auto the locale substitutes a word where it has one: English yields yesterday, today, tomorrow, last week, last quarter. Other languages go further than English can — Spanish returns anteayer for two days ago, a single word for a concept English needs three. That is the argument for auto in a multilingual interface: you get each language's idiom instead of a translated English sentence.

The API also handles plural agreement, which is the part hand-rolled formatters always get wrong. Russian needs 2 дня назад but 5 дней назад; Polish, Arabic and Welsh have their own category systems. A template that interpolates a number into a translated string cannot express that, and it is why this formatter exists in the platform rather than in your codebase.

One thing it deliberately does not do is choose the unit. You pass a value and a unit, and it formats exactly what you passed — including fractions, so -1.5 with day comes out as 1.5 days ago. Picking and rounding the unit is your code's job, and the honest version of that job has a subtlety: yesterday in most interfaces means the previous calendar date, not 24 hours ago. At 01:00 on Tuesday, an event at 23:00 on Monday is two hours old by arithmetic, and the formatter will say so. If you want it to say yesterday, compare calendar dates in the user's time zone before deciding the unit — the table here shows the arithmetic answer for every unit, which makes that mismatch easy to see.