SwiftYMD helps to calculate dates in the forms of YMD and YM.
let ymd1 = YMD(1988, 12, 3)! // Dec. 3, 1988
let ymd2 = ymd1 + .days(11331) // Dec. 12, 2012
print(ymd2 - ymd1) // 11331
print(ymd2.dayOfWeek) // Thursday
for ymd in YMD(1988, 12, 3)! ... YMD(2019, 12, 12)! {
// ...
}