fix(date): compute %U and %W week numbers like strftime (#958)

src/util/strftime.spec.ts

@@ -140,6 +140,22 @@ describe('util/strftime', function () {
     it('should format %W as week of year, starts with 1', function () {
       expect(t(now, '%W')).toBe('01')
     })
+    it('should format %U and %W as 00 before the first Sunday/Monday', function () {
+      // 2021-01-01 is a Friday
+      expect(t(new TestDate('2021-01-01 12:00:00'), '%U %W')).toBe('00 00')
+      expect(t(new TestDate('2021-01-03 12:00:00'), '%U %W')).toBe('01 00')
+      expect(t(new TestDate('2021-01-04 12:00:00'), '%U %W')).toBe('01 01')
+    })
+    it('should keep Sunday in the previous %W week', function () {
+      // 2000-01-02 is a Sunday
+      expect(t(new TestDate('2000-01-02 12:00:00'), '%W')).toBe('00')
+      expect(t(new TestDate('2000-01-09 12:00:00'), '%W')).toBe('01')
+    })
+    it('should format %U and %W when January 1st is the start day', function () {
+      // 2006-01-01 is a Sunday, 2007-01-01 is a Monday
+      expect(t(new TestDate('2006-07-05 12:00:00'), '%U %W')).toBe('27 27')
+      expect(t(new TestDate('2007-01-01 12:00:00'), '%U %W')).toBe('00 01')
+    })
   })
 
   describe('Time zone', () => {

src/util/strftime.ts

@@ -23,12 +23,9 @@ function getDayOfYear (d: LiquidDate) {
   return num + d.getDate()
 }
 function getWeekOfYear (d: LiquidDate, startDay: number) {
-  // Skip to startDay of this week
+  // Days before the first startDay of the year belong to week 0
-  const now = getDayOfYear(d) + (startDay - d.getDay())
+  const daysSinceStartDay = (d.getDay() - startDay + 7) % 7
-  // Find the first startDay of the year
+  return String(Math.floor((getDayOfYear(d) - 1 - daysSinceStartDay + 7) / 7))
-  const jan1 = new Date(d.getFullYear(), 0, 1)
-  const then = (7 - jan1.getDay() + startDay)
-  return String(Math.floor((now - then) / 7) + 1)
 }
 function isLeapYear (d: LiquidDate) {
   const year = d.getFullYear()