Swift中日期之間的天數

Question

我在Swift的Calendar dateComponents（_：from：to :)函數中發現了一個非常奇怪的行為。

let a = Calendar.current.date(byAdding: DateComponents(day: 7), to: Date())!
let n1 = Calendar.current.dateComponents([.day], from: Date(), to: a).day!
let n2 = Calendar.current.dateComponents([.day], from: Date(), to: Calendar.current.date(byAdding: DateComponents(day: 7), to: Date())!).day!
print(a)
print(Calendar.current.date(byAdding: DateComponents(day: 7), to: Date())!)
print(n1)
print(n2)

打印：

2018-04-21 19:58:16 +0000
2018-04-21 19:58:16 +0000
6
7

我想念什么？

Answer 1

正如Martin在他的評論中所說，問題在於您在每個步驟上都獲得了一個新的Date（）值，而最后一個值要比第一個值晚一秒鍾。

請注意，此代碼給出了預期的輸出：

let now = Date()
let a = Calendar.current.date(byAdding: DateComponents(day: 7), to: now)!
let n1 = Calendar.current.dateComponents([.day], from: now, to: a).day!
let n2 = Calendar.current.dateComponents([.day], from: now, to: Calendar.current.date(byAdding: DateComponents(day: 7), to: now)!).day!
print(a)
print(Calendar.current.date(byAdding: DateComponents(day: 7), to: now)!)
print(n1)
print(n2)

之所以有效，是因為所有日期都是完全相同的值，而不是相差幾分之一秒的值。