2 files changed, 98 insertions, 1 deletions
diff --git a/src/utils/datetime.test.ts b/src/utils/datetime.test.ts
index 5f0749d..03e2771 100644
--- a/src/utils/datetime.test.ts
+++ b/src/utils/datetime.test.ts
@@ -1,6 +1,10 @@
 import { assertEquals, assertMatch } from "@std/assert";
 import { describe, it } from "@std/testing/bdd";
-import { toIso8601Full, toIso8601FullUTC } from "./datetime.ts";
+import {
+  getRelativeTimeUnit,
+  toIso8601Full,
+  toIso8601FullUTC,
+} from "./datetime.ts";
 
 describe("toIso8601Full", () => {
   it("formats current local time with offset", () => {
@@ -60,3 +64,71 @@ describe("toIso8601FullUTC", () => {
     assertMatch(result, /^-\d{6}-03-15T12:00:00\.000Z$/);
   });
 });
+
+describe("getRelativeTimeUnit", () => {
+  const now = new Date();
+
+  it("returns seconds for differences under a minute", () => {
+    const future = new Date(now.getTime() + 45 * 1000);
+    const [value, unit] = getRelativeTimeUnit(future);
+
+    assertEquals(unit, "second");
+    assertEquals(value, 1);
+  });
+
+  it("returns minutes for differences under an hour but over a minute", () => {
+    const future = new Date(now.getTime() + 15 * 60 * 1000);
+    const [value, unit] = getRelativeTimeUnit(future);
+
+    assertEquals(unit, "minute");
+    assertEquals(value, 15);
+  });
+
+  it("returns hours for differences under a day but over an hour", () => {
+    const past = new Date(now.getTime() - 3.2 * 60 * 60 * 1000);
+    const [value, unit] = getRelativeTimeUnit(past);
+
+    assertEquals(unit, "hour");
+    assertEquals(value, -3);
+  });
+
+  it("returns days for differences under a week but over a day", () => {
+    const past = new Date(now.getTime() - 2.5 * 24 * 60 * 60 * 1000);
+    const [value, unit] = getRelativeTimeUnit(past);
+
+    assertEquals(unit, "day");
+    assertEquals(value, -3);
+  });
+
+  it("returns weeks for differences under a month but over a week", () => {
+    const future = new Date(now.getTime() + 2.3 * 7 * 24 * 60 * 60 * 1000);
+    const [value, unit] = getRelativeTimeUnit(future);
+
+    assertEquals(unit, "week");
+    assertEquals(value, 2);
+  });
+
+  it("returns months for differences under a quarter but over a month", () => {
+    const past = new Date(now.getTime() - 1.5 * 30 * 24 * 60 * 60 * 1000);
+    const [value, unit] = getRelativeTimeUnit(past);
+
+    assertEquals(unit, "month");
+    assertEquals(value, -2);
+  });
+
+  it("returns quarters for differences under a year but over a quarter", () => {
+    const future = new Date(now.getTime() + 5 * 30 * 24 * 60 * 60 * 1000);
+    const [value, unit] = getRelativeTimeUnit(future);
+
+    assertEquals(unit, "quarter");
+    assertEquals(value, 2);
+  });
+
+  it("returns years for differences over a year", () => {
+    const past = new Date(now.getTime() - 3.4 * 365 * 24 * 60 * 60 * 1000);
+    const [value, unit] = getRelativeTimeUnit(past);
+
+    assertEquals(unit, "year");
+    assertEquals(value, -3);
+  });
+});
diff --git a/src/utils/datetime.ts b/src/utils/datetime.ts
index 3a2cd25..c32fde0 100644
--- a/src/utils/datetime.ts
+++ b/src/utils/datetime.ts
@@ -41,3 +41,28 @@ export function toIso8601FullUTC(date: Date): string {
 }
 
 const pad = (num: number, len = 2) => String(Math.abs(num)).padStart(len, "0");
+
+export function getRelativeTimeUnit(
+  date: Date,
+  now: Date = new Date(),
+): Parameters<Intl.RelativeTimeFormat["format"]> {
+  const diffMs = date.getTime() - now.getTime();
+
+  const seconds = diffMs / 1000;
+  const minutes = seconds / 60;
+  const hours = minutes / 60;
+  const days = hours / 24;
+  const weeks = days / 7;
+  const months = days / 30;
+  const quarters = months / 3;
+  const years = days / 365;
+
+  if (Math.abs(years) >= 1) return [Math.round(years), "year"];
+  if (Math.abs(quarters) >= 1) return [Math.round(quarters), "quarter"];
+  if (Math.abs(months) >= 1) return [Math.round(months), "month"];
+  if (Math.abs(weeks) >= 1) return [Math.round(weeks), "week"];
+  if (Math.abs(days) >= 1) return [Math.round(days), "day"];
+  if (Math.abs(hours) >= 1) return [Math.round(hours), "hour"];
+  if (Math.abs(minutes) >= 1) return [Math.round(minutes), "minute"];
+  return [Math.round(seconds), "second"];
+}