快速计算两个日期之间的天数 [英] Calculating the number of days between two dates in swift

问题描述

为什么在计算两个日期之间的天数时使用变量 today 而不是 Date() 的结果会在 swift 中产生差异?

Why does the result of using the variable today rather than Date() when calculating the number of days between two dates make a difference in swift?

var numDays: Int
var today = Date()
var twoWeeksFromNow: Date = Calendar.current.date(byAdding: .day, 
value: 14, to: Date())!

var numDaysWithDate: Int = 
Calendar.current.dateComponents([.day], from: Date(), to: 
twoWeeksFromNow).day!
var numDaysWithToday: Int = 
Calendar.current.dateComponents([.day], from: today, to: 
twoWeeksFromNow).day!

print(numDaysWithDate) // 13
print(numDaysWithToday) // 14

推荐答案

这与您创建变量并比较它们的顺序有关.

It has to do with the order that you're creating your variables and comparing them.

您要做的第一件事是创建今天.然后，您创建 twoWeeksFromNow，基于一个新的 Date()，它将在未来比 今天稍微> 是.在我的机器上，在操场上，第二次约会比第一次晚了大约 300 微秒.

The first thing you do is create today. Then, you create twoWeeksFromNow, based on a new Date() that will be slightly further in the future than today was. On my machine, in a playground, the second date is about 300 microseconds further in the future than the first.

然后，对于numDaysWithDate，您将twoWeeksFromNow 与另一个 新的Date() 进行比较，甚至更多稍微在未来.因此，您的时间框架非常不到整整 2 周，给您 13 天.

Then, for numDaysWithDate, you compare twoWeeksFromNow to another new Date(), even more slightly in the future. So, your time frame is very slightly less than 2 full weeks, giving you 13 days.

但是，对于 numDaysWithToday，您将 twoWeeksFromNow 与原始 today 进行比较，后者是 before >twoWeeksFromNow 是，让它稍微超过 2 周，给你 14 天.

But, for numDaysWithToday, you compare twoWeeksFromNow with the original today, which was created before twoWeeksFromNow was, making it slightly longer than 2 weeks, giving you 14 days.

如果您更改 today 和 twoWeeksFromNow 声明的顺序，您会看到不同的结果:

If you change the order of the the today and twoWeeksFromNow declarations, you can see a different result:

var twoWeeksFromNow: Date = Calendar.current.date(byAdding: .day,
value: 14, to: Date())!
var today = Date()

现在，因为 today 的创建日期比 twoWeeksFromNow 的创建日期稍晚，所以两个结果都是 13.

Now, because today was created slightly later than the date that twoWeeksFromNow was created from, both results are 13.

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