fix(date): compute %C and %y from the numeric year (#961)

Both were sliced from the year string, so years with other than four
digits gave the wrong century and two-digit year, and %y was a string
that the - and _ flags could not unpad. Compute them arithmetically and
pad them to two digits, matching Ruby's strftime.

src/util/strftime.spec.ts

@@ -18,6 +18,14 @@ describe('util/strftime', function () {
     it('should format %y as 2-digit year', function () {
       expect(t(now, '%y')).toBe('16')
     })
+    it('should format %C and %y from the numeric year', function () {
+      const year999 = new TestDate('0999-06-15 00:00:00')
+      expect(t(year999, '%C')).toBe('09')
+      expect(t(year999, '%y')).toBe('99')
+      const year2005 = new TestDate('2005-06-15 00:00:00')
+      expect(t(year2005, '%y')).toBe('05')
+      expect(t(year2005, '%-y')).toBe('5')
+    })
     describe('%j', function () {
       it('should format %j as day of year', function () {
         expect(t(then, '%j')).toBe('066')