user2228656 - 8 months ago 68

Swift Question

I am trying to study

`NSDate`

I found that each NSDate object has three properties, which are:

- timeIntervalSinceReferenceDate
- timeIntervalSince1970
- timeIntervalSinceNow

A little bit confusing for me. I couldn't understand the interpretation of them though I know they represents the seconds.

I did this code:

`let whatISThis = NSDate(timeIntervalSinceReferenceDate: NSTimeInterval(60))`

Okay I got a date, but what is that? what is the difference between those three properties?

Answer

`timeIntervalSinceReferenceDate`

is the number of seconds since January, 1st, 2001: 12:00 am (mid night)

`timeIntervalSince1970`

is the number of seconds since January, 1st, 1970, 12:00 am (mid night)

`timeIntervalSinceNow`

is the number of seconds since now

I will list these examples:

```
let s0 = NSDate(timeIntervalSinceReferenceDate: NSTimeInterval(0)) // it means give me the time that happens after January,1st, 2001, 12:00 am by zero seconds
print("\(s0)") //2001-01-01 00:00:00
let s60 = NSDate(timeIntervalSinceReferenceDate: NSTimeInterval(60)) //it means give me the time that happens after January, 1st, 2001, 12:00 am by **60 seconds**
print("\(s60)") //2001-01-01 00:01:00
let s2 = NSDate(timeIntervalSince1970: NSTimeInterval(0)) // it means give me the time that happens after January, 1st, 1970 12:00 am by **zero** seconds
print("\(s2)") //1970-01-01 00:00:00
let s3 = NSDate() // it means the current time
print("\(s3)")//2015-10-25 14:12:40
let s4 = NSDate(timeIntervalSinceNow: NSTimeInterval(60)) //it means one minute (60 seconds) after the current time
print("\(s4)") //2015-10-25 14:13:40
let s5 = NSDate(timeIntervalSinceNow: NSTimeInterval(-60)) // it means one minute (60 seconds) before the current time
print("\(s5)") //2015-10-25 14:11:40
let sd = NSDate(timeIntervalSinceReferenceDate: NSTimeInterval(60)) // it means one minute after the reference time (January, 1st, 1970: 12:00 am)
print("\(sd)") //2001-01-01 00:01:00
```

And of course if you have a NSDate objects, you can take all these properties simply ...

```
let sNow = NSDate()
sNow.timeIntervalSinceReferenceDate
sNow.timeIntervalSinceNow
sNow.timeIntervalSince1970
```