import Cocoa

for dc in [
    DateComponents(calendar: .current, year: 2018, month: 12, day: 25),
    DateComponents(calendar: .current, year: 2019, month: 1, day: 1)
]
{
    print(
        Calendar.current.dateComponents(
            [.weekOfYear],
            from: Calendar.current.date(from: dc)!
        ).weekOfYear
    )
}