Use cache-friendly "time ago"

Also introduces a local time helper that we can use to format dates and
times.

app/javascript/controllers/local_time_controller.js

@@ -0,0 +1,62 @@
+import { Controller } from "@hotwired/stimulus"
+
+export default class extends Controller {
+  static targets = ["time", "date", "datetime", "ago"]
+
+  initialize() {
+    this.timeFormatter = new Intl.DateTimeFormat(undefined, { timeStyle: "short" })
+    this.dateFormatter = new Intl.DateTimeFormat(undefined, { dateStyle: "long" })
+    this.dateTimeFormatter = new Intl.DateTimeFormat(undefined, { timeStyle: "short", dateStyle: "short" })
+    this.agoFormatter = new AgoFormatter()
+  }
+
+  timeTargetConnected(target) {
+    this.#formatTime(this.timeFormatter, target)
+  }
+
+  dateTargetConnected(target) {
+    this.#formatTime(this.dateFormatter, target)
+  }
+
+  datetimeTargetConnected(target) {
+    this.#formatTime(this.dateTimeFormatter, target)
+  }
+
+  agoTargetConnected(target) {
+    this.#formatTime(this.agoFormatter, target)
+  }
+
+  #formatTime(formatter, target) {
+    const dt = new Date(target.getAttribute("datetime"))
+    target.textContent = formatter.format(dt)
+    target.title = this.dateTimeFormatter.format(dt)
+  }
+}
+
+class AgoFormatter {
+  format(dt) {
+    const now = new Date()
+    const seconds = (now - dt) / 1000
+    const minutes = seconds / 60
+    const hours = minutes / 60
+    const days = hours / 24
+    const weeks = days / 7
+    const months = days / (365 / 12)
+    const years = days / 365
+
+    if (years >= 1) return this.#pluralize("year", years)
+    if (months >= 1) return this.#pluralize("month", months)
+    if (weeks >= 1) return this.#pluralize("week", weeks)
+    if (days >= 1) return this.#pluralize("day", days)
+    if (hours >= 1) return this.#pluralize("hour", hours)
+    if (minutes >= 1) return this.#pluralize("minute", minutes)
+
+    return "Less than a minute ago"
+  }
+
+  #pluralize(word, quantity) {
+    quantity = Math.round(quantity)
+    const suffix = (quantity === 1) ? "" : "s"
+    return `${quantity} ${word}${suffix} ago`
+  }
+}