斯威夫特的日期之间的天数

Question

我在Swift的日历dateComponents（_：from：to :)函数中发现了一个非常奇怪的行为。

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

我错过了什么？

Answer 1

正如Martin在评论中所说，问题是你在每一步获得了一个新的Date（）值，而最后一个值比第一个值迟了一小部分。

请注意，此代码提供了预期的输出：

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)

它的工作原理是因为所有日期都是完全相同的值，而不是相差几分之一的值。