Swiftで旧暦を扱う ref: https://qiita.com/SatoTakeshiX/items/f58ed3fb9d8beb16273b

file0.swift

let today = Date() //2018年12月18日に実行
// 中国の農暦 = 旧暦
let chineseCalendar = Calendar(identifier: .chinese)
let comp = chineseCalendar.dateComponents([.year, .month, .day], from: today)
print(comp.debugDescription)
// year: 35 month: 11 day: 12 isLeapMonth: false 

file1.swift

// グレゴリオ暦 = 西暦
let gregrianCalendar = Calendar(identifier: .gregorian)
var compornents = DateComponents()
compornents.year = 2018
compornents.month = 12
compornents.day = 18

// 西暦からDate型を作る
let gregrianDate = gregrianCalendar.date(from: compornents)!

// 中国の農暦 = 旧暦
let chineseCalendar = Calendar(identifier: .chinese)
// 旧暦に変換
let chineseComps = chineseCalendar.dateComponents([.year, .month, .day], from: gregrianDate)
print(chineseComps.debugDescription)