Format a timestamp as relative time

Every feed, comment thread and notification list shows relative times. Passing now in explicitly rather than reading the clock inside is what makes the function testable — and it is why this version is deterministic.

Implement formatRelativeTime(timestamp, now), both epoch milliseconds. Let seconds = Math.round((now - timestamp) / 1000) — positive means the past — and abs = Math.abs(seconds).

If abs is under 60, return exactly "just now". Otherwise pick the first unit whose limit abs falls under, and take Math.floor(abs / size) as the value: minute (size 60, limit 3600), hour (3600, 86400), day (86400, 604800), week (604800, 2592000), month (2592000, 31536000), and otherwise year (size 31536000).

Build the label as the value, a space, the unit name, plus an s unless the value is exactly 1. Return "<label> ago" when seconds is positive and "in <label>" when it is negative.

What it has to do

• Return "just now" when the difference is under 60 seconds in either direction.
• Use the largest unit that fits, flooring the value, across minute, hour, day, week, month and year.
• Pluralise the unit with a trailing s unless the value is exactly 1.
• Suffix past times with " ago" and prefix future times with "in ".
• Take now as an argument rather than reading the clock.

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