1 files changed, 24 insertions, 0 deletions
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
  */