3 files changed, 47 insertions, 4 deletions
diff --git a/std/datetime/README.md b/std/datetime/README.md
index 6c29f65e4..ae6b6f133 100644
--- a/std/datetime/README.md
+++ b/std/datetime/README.md
@@ -37,3 +37,13 @@ import {
 dayOfYear(new Date("2019-03-11T03:24:00")); // output: 70
 currentDayOfYear(); // output: ** depends on when you run it :) **
 ```
+
+### weekOfYear
+
+- `weekOfYear(date: Date)` - Returns the week number of the provided date (1-53)
+
+```ts
+import { weekOfYear } from "https://deno.land/std/datetime/mod.ts";
+
+weekOfYear(new Date("2020-12-28T03:24:00")); // Returns 53
+```
diff --git a/std/datetime/mod.ts b/std/datetime/mod.ts
index 99332e046..426464564 100644
--- a/std/datetime/mod.ts
+++ b/std/datetime/mod.ts
@@ -111,6 +111,30 @@ export function dayOfYear(date: Date): number {
 }
 
 /**
+ * Get number of the week in the year (ISO-8601)
+ * @return Number of the week in year
+ */
+export function weekOfYear(date: Date): number {
+  const workingDate = new Date(
+    Date.UTC(date.getFullYear(), date.getMonth(), date.getDate())
+  );
+
+  // Set to nearest Thursday: current date + 4 - current day number
+  // Make Sunday's day number 7
+  workingDate.setUTCDate(
+    workingDate.getUTCDate() + 4 - (workingDate.getUTCDay() || 7)
+  );
+
+  // Get first day of year
+  const yearStart = new Date(Date.UTC(workingDate.getUTCFullYear(), 0, 1));
+
+  // return the calculated full weeks to nearest Thursday
+  return Math.ceil(
+    ((workingDate.valueOf() - yearStart.valueOf()) / 86400000 + 1) / 7
+  );
+}
+
+/**
  * Get number of current day in year
  * @return Number of current day in year
  */
diff --git a/std/datetime/test.ts b/std/datetime/test.ts
index dc4c7278e..ef4b5dd58 100644
--- a/std/datetime/test.ts
+++ b/std/datetime/test.ts
@@ -81,16 +81,25 @@ Deno.test({
 Deno.test({
   name: "[std/datetime] DayOfYear",
   fn: () => {
-    assertEquals(1, datetime.dayOfYear(new Date("2019-01-01T03:24:00")));
-    assertEquals(70, datetime.dayOfYear(new Date("2019-03-11T03:24:00")));
-    assertEquals(365, datetime.dayOfYear(new Date("2019-12-31T03:24:00")));
+    assertEquals(datetime.dayOfYear(new Date("2019-01-01T03:24:00")), 1);
+    assertEquals(datetime.dayOfYear(new Date("2019-03-11T03:24:00")), 70);
+    assertEquals(datetime.dayOfYear(new Date("2019-12-31T03:24:00")), 365);
   },
 });
 
 Deno.test({
   name: "[std/datetime] currentDayOfYear",
   fn: () => {
-    assertEquals(datetime.currentDayOfYear(), datetime.dayOfYear(new Date()));
+    assertEquals(datetime.dayOfYear(new Date()), datetime.currentDayOfYear());
+  },
+});
+
+Deno.test({
+  name: "[std/datetime] WeekOfYear",
+  fn: () => {
+    assertEquals(datetime.weekOfYear(new Date("2020-01-05T03:24:00")), 1);
+    assertEquals(datetime.weekOfYear(new Date("2020-12-28T03:24:00")), 53); // 53 weeks in 2020
+    assertEquals(datetime.weekOfYear(new Date("2020-06-28T03:24:00")), 26);
   },
 });