Fix: ensure correct relative dates formatter uses calendar days (#7174)

next-i18next.config.js

@@ -106,6 +106,12 @@ function relativeDate(date, formatter) {
   const unitIndex = cutoffs.findIndex((cutoff) => cutoff > Math.abs(delta));
   const divisor = unitIndex ? cutoffs[unitIndex - 1] : 1;
 
+  if (units[unitIndex] === "day") {
+    // compare calendar days, not elapsed 24h blocks
+    const startOfDay = (d) => new Date(d.getFullYear(), d.getMonth(), d.getDate());
+    return formatter.format(Math.round((startOfDay(date) - startOfDay(new Date())) / 86400000), "day");
+  }
+
   return formatter.format(Math.floor(delta / divisor), units[unitIndex]);
 }
 

src/utils/relative-date-formatter.test.js

@@ -0,0 +1,46 @@
+import { createRequire } from "node:module";
+
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+const require = createRequire(import.meta.url);
+const i18nextConfig = require("../../next-i18next.config.js");
+
+function getRelativeDateFormatter() {
+  let relativeDateFormatter;
+
+  i18nextConfig.use[0].init({
+    services: {
+      formatter: {
+        add(name, formatter) {
+          if (name === "relativeDate") relativeDateFormatter = formatter;
+        },
+      },
+    },
+  });
+
+  return relativeDateFormatter;
+}
+
+describe("relativeDate formatter", () => {
+  const formatRelativeDate = getRelativeDateFormatter();
+  const options = { numeric: "auto" };
+
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date(2026, 8, 23, 16, 0));
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it.each([
+    ["hours away on the same day", new Date(2026, 8, 23, 20, 0), "in 4 hours"],
+    ["next calendar day", new Date(2026, 8, 24, 20, 0), "tomorrow"],
+    ["two calendar days away but under 48h", new Date(2026, 8, 25, 10, 40), "in 2 days"],
+    ["previous calendar day", new Date(2026, 8, 22, 10, 0), "yesterday"],
+    ["two calendar days ago but under 48h", new Date(2026, 8, 21, 20, 0), "2 days ago"],
+  ])("formats %s", (_description, value, expected) => {
+    expect(formatRelativeDate(value.toISOString(), "en", options)).toBe(expected);
+  });
+});