diff --git a/std/datetime/README.md b/std/datetime/README.md
index ae6b6f133e..e9142f48ad 100644
--- a/std/datetime/README.md
+++ b/std/datetime/README.md
@@ -40,7 +40,7 @@ currentDayOfYear(); // output: ** depends on when you run it :) **
 
 ### weekOfYear
 
-- `weekOfYear(date: Date)` - Returns the week number of the provided date (1-53)
+Returns the ISO week number of the provided date (1-53)
 
 ```ts
 import { weekOfYear } from "https://deno.land/std/datetime/mod.ts";
diff --git a/std/datetime/mod.ts b/std/datetime/mod.ts
index 4264645646..9665a92b29 100644
--- a/std/datetime/mod.ts
+++ b/std/datetime/mod.ts
@@ -8,6 +8,17 @@ export const MINUTE = SECOND * 60;
 export const HOUR = MINUTE * 60;
 export const DAY = HOUR * 24;
 export const WEEK = DAY * 7;
+const DAYS_PER_WEEK = 7;
+
+enum Day {
+  Sun,
+  Mon,
+  Tue,
+  Wed,
+  Thu,
+  Fri,
+  Sat,
+}
 
 function execForce(reg: RegExp, pat: string): RegExpExecArray {
   const v = reg.exec(pat);
@@ -110,6 +121,14 @@ export function dayOfYear(date: Date): number {
   return Math.floor(diff / DAY);
 }
 
+/**
+ * Get number of current day in year
+ * @return Number of current day in year
+ */
+export function currentDayOfYear(): number {
+  return dayOfYear(new Date());
+}
+
 /**
  * Get number of the week in the year (ISO-8601)
  * @return Number of the week in year
@@ -119,27 +138,20 @@ export function weekOfYear(date: Date): number {
     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)
-  );
+  const day = workingDate.getUTCDay();
+
+  const nearestThursday =
+    workingDate.getUTCDate() +
+    Day.Thu -
+    (day === Day.Sun ? DAYS_PER_WEEK : day);
+
+  workingDate.setUTCDate(nearestThursday);
 
   // 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
- */
-export function currentDayOfYear(): number {
-  return dayOfYear(new Date());
+  return Math.ceil((workingDate.getTime() - yearStart.getTime() + DAY) / WEEK);
 }
 
 /**
diff --git a/std/datetime/test.ts b/std/datetime/test.ts
index ef4b5dd58e..a223abce22 100644
--- a/std/datetime/test.ts
+++ b/std/datetime/test.ts
@@ -79,7 +79,7 @@ Deno.test({
 });
 
 Deno.test({
-  name: "[std/datetime] DayOfYear",
+  name: "[std/datetime] dayOfYear",
   fn: () => {
     assertEquals(datetime.dayOfYear(new Date("2019-01-01T03:24:00")), 1);
     assertEquals(datetime.dayOfYear(new Date("2019-03-11T03:24:00")), 70);
@@ -95,11 +95,65 @@ Deno.test({
 });
 
 Deno.test({
-  name: "[std/datetime] WeekOfYear",
+  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);
+
+    // iso weeks year starting sunday
+    assertEquals(datetime.weekOfYear(new Date(2012, 0, 1)), 52);
+    assertEquals(datetime.weekOfYear(new Date(2012, 0, 2)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2012, 0, 8)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2012, 0, 9)), 2);
+    assertEquals(datetime.weekOfYear(new Date(2012, 0, 15)), 2);
+
+    // iso weeks year starting monday
+    assertEquals(datetime.weekOfYear(new Date(2007, 0, 1)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2007, 0, 7)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2007, 0, 8)), 2);
+    assertEquals(datetime.weekOfYear(new Date(2007, 0, 14)), 2);
+    assertEquals(datetime.weekOfYear(new Date(2007, 0, 15)), 3);
+
+    // iso weeks year starting tuesday
+    assertEquals(datetime.weekOfYear(new Date(2007, 11, 31)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2008, 0, 1)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2008, 0, 6)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2008, 0, 7)), 2);
+    assertEquals(datetime.weekOfYear(new Date(2008, 0, 13)), 2);
+    assertEquals(datetime.weekOfYear(new Date(2008, 0, 14)), 3);
+
+    // iso weeks year starting wednesday
+    assertEquals(datetime.weekOfYear(new Date(2002, 11, 30)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2003, 0, 1)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2003, 0, 5)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2003, 0, 6)), 2);
+    assertEquals(datetime.weekOfYear(new Date(2003, 0, 12)), 2);
+    assertEquals(datetime.weekOfYear(new Date(2003, 0, 13)), 3);
+
+    // iso weeks year starting thursday
+    assertEquals(datetime.weekOfYear(new Date(2008, 11, 29)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2009, 0, 1)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2009, 0, 4)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2009, 0, 5)), 2);
+    assertEquals(datetime.weekOfYear(new Date(2009, 0, 11)), 2);
+    assertEquals(datetime.weekOfYear(new Date(2009, 0, 13)), 3);
+
+    // iso weeks year starting friday
+    assertEquals(datetime.weekOfYear(new Date(2009, 11, 28)), 53);
+    assertEquals(datetime.weekOfYear(new Date(2010, 0, 1)), 53);
+    assertEquals(datetime.weekOfYear(new Date(2010, 0, 3)), 53);
+    assertEquals(datetime.weekOfYear(new Date(2010, 0, 4)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2010, 0, 10)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2010, 0, 11)), 2);
+
+    // iso weeks year starting saturday
+    assertEquals(datetime.weekOfYear(new Date(2010, 11, 27)), 52);
+    assertEquals(datetime.weekOfYear(new Date(2011, 0, 1)), 52);
+    assertEquals(datetime.weekOfYear(new Date(2011, 0, 2)), 52);
+    assertEquals(datetime.weekOfYear(new Date(2011, 0, 3)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2011, 0, 9)), 1);
+    assertEquals(datetime.weekOfYear(new Date(2011, 0, 10)), 2);
   },
 });