test_that("iso", {
  dec25 <- as_iso(gregorian_date(2025, 12, 1:31))
  expect_equal(
    gregorian_date(1967, 5, 2) |>
      as_iso() |>
      cal_iso$from_rd() |>
      cal_iso$to_rd(),
    gregorian_date(1967, 5, 2) |> as.numeric()
  )
  expect_no_error(as_iso(1:1e5) |> as.list() |> validate_iso())

  expect_equal(week_of_year(dec25), granularity(dec25, "week"))
  expect_equal(
    day_of_year(dec25)[1:28],
    day_of_year(as_gregorian(dec25))[1:28] + 2
  )
})