fix: parse ISO dates with a year before 1000

src/__tests__/parse.spec.ts

@@ -278,4 +278,12 @@ describe("parse", () => {
       ).toISOString()
     ).toBe("1989-12-19T07:30:10.000Z")
   })
+  it("parses an ISO date whose year is before 1000", () => {
+    // Regression: the `four` helper padded to 2 digits, so a valid
+    // zero-padded year like "0087" produced an invalid ISO string and
+    // parse() threw instead of returning the date.
+    const d = parse("0087-06-15", "YYYY-MM-DD")
+    expect(Number.isNaN(+d)).toBe(false)
+    expect(d.getUTCFullYear()).toBe(87)
+  })
 })

src/common.ts

@@ -159,7 +159,7 @@ export const two = (n: number) => String(n).padStart(2, "0")
  * Creates a leading zero string of 4 digits.
  * @param n - A number.
  */
-export const four = (n: number) => String(n).padStart(2, "0")
+export const four = (n: number) => String(n).padStart(4, "0")
 
 /**
  * Normalizes a given part to NFKC.