let calendar = Calendar.utc

// 1970/01/01(thu) 00:00 +00:00
let date = Date(timeIntervalSince1970: 0)

// TimeKit provides date components: `Day`, `Month`, `Year`, `Weekday`.
// These are compatible with `Date`.
let day = calendar.day(for: date)
let month = calendar.month(for: date)
let year = calendar.year(for: date)
let weekday = day.weekday

let inUTC = Calendar.utc.startDate(of: day)           // 1970/01/01(thu) 00:00 +00:00
let inTokyo = Calendar.tokyo.startDate(of: day)       // 1970/01/01(thu) 09:00 +00:00
let inHonolulu = Calendar.honolulu.startDate(of: day) // 1969/12/31(wed) 14:00 +00:00

let calendar = Calendar.utc
let date = Date(timeIntervalSince1970: 0)

let day = calendar.day(for: date)
let month = calendar.month(for: date)
let year = calendar.year(for: date)

let startOfDay = calendar.start(of: day)
let startOfMonth = calendar.start(of: month)
let startOfYear = calendar.start(of: year)

// A day after / before `today`
let tomorrow = day.next
let yesterday = day.previous

// Addition and subtraction
let threeDaysLater = day + 3.day
let fourMonthsAgo = month - 4.month
let fiveYearsLater = year + 5.year

// Obtain the date components range
let days = 42.days(from: today)     // ["1970/01/01", "1970/01/02", "1970/01/03", ...]
let months = 12.months(from: month) // ["1970/01", "1970/02", "1970/03", ...]
let years = 3.years(from: year)     // ["1970", "1971", "1972"]

let dateFormatter = ...

let dayString = dateFormatter.string(from: day)
print(dayString) // => "1970/01/01(thu)"

let day = dateFormatter.day(from: "1970/01/01(thu)")
print(day) // => "1970/01/01(thu) in gregorian calendar"