Determine date range given year and week number

[hhaensel]

@JeffreySarnoff

function firstmondayofISOyear(year)
  date = Date(Year(year))
  firstthursdayofyear = firstdayofweek(date) + Day(3)
  yr = Year(firstthursdayofyear).value
  if yr < year
    firstthursdayofyear += Day(7)
  end
  firstthursdayofyear - Day(3)
end

If I should propose a solution for Dates.jl, I’d go for a typestable definition

function firstdayofISOyear(x)
  firstday = firstdayofweek(firstdayofyear(x))
  week(firstday) == 1 ? firstday : firstday + Day(7)
end

firstdayofISOyear(x::Integer) = firstdayofISOyear(Date(Year(x)))
firstdayofISOweek(year, week::Integer) = firstdayofISOyear(year) + Day(7(week - 1))

julia> firstdayofISOyear(2025)
2024-12-30

julia> firstdayofISOyear(today())
2025-03-31

julia> firstdayofISOyear(now())
2025-03-31T00:00:00

julia> firstdayofISOyear(2005)
2005-01-03


julia> firstdayofISOweek(2005, 43)
2005-10-24

julia> firstdayofISOweek(now(), 43)
2025-10-20T00:00:00

Any opinions to this version?