Tag Archives: NSDateComponents

How to add some time to an NSDate

Imagine you had an NSDate object and want to add days several to it. We can use NSDate method dateByAddingComponents for this, which takes – as you may have guessed – NSDateComponents as parameters.

In this example, let’s assume we want to know what date it is 5 days from today:

You can add as many and diverse components you like, such as

  • setEra:
  • setYear:
  • setMonth:
  • setDay:
  • setHour:
  • setMinute:
  • setSecond:
  • setWeek:
  • setWeekday:
  • setWeekdayOrdinal:
  • setQuarter:
  • setCalendar:
  • setTimeZone:
  • setWeekOfMonth:
  • setWeekOfYear:
  • setYearForWeekOfYear:

Check out the NSDateComponents Class Reference for more information.

How to normalize NSDate objects (i.e. set the time to midnight)

When you create new date using [NSDate date] (i.e. today, as in right now) your date will save the current time as well as its date.

In fact, under the hood an NSDate object is the amount of seconds that have elapsed since the 1st of January 2001 (or 1970), in milliseconds. So really it’s a massive floating point number. You can see what it is with this code:

To save you the trouble, it’s bee roughly 1362585768.752258 seconds since 1970. But I digress…

Sometimes you want the date objects you’re dealing with not to regard “time” and really only deal with dates. As such, all dates – to be comparable – need to have the very same time, say midnight. Otherwise you may get the wrong results. Imagine adding three date objects that each have noon as their time, that’s an extra day and a half clouding your results right there.

Here’s some code that will normalize your NSDate object by setting its time to midnight:

In a nutshell, we “explode” the date into its year, month and day components, and then recreate a new object with these and no other components.

How to determine how many days / months / years have passed between two NSDate objects

Imagine you had two NSDate objects and you’d like to find out the time interval between those dates. NSDate objects alone won’t help us out there unless we do some serious NSDateFormatting and hair pulling.

Lucky for us there are a few other classes available that will help us do this, namely NSCalendar and NSDateComponents.

 

Years, Months and Days

Here’s how you can determine how many years, months and days have passed between two NSDates:

The above example returns Years: 12, Months: 2, Days: 5.

In a nutshell we split the date into “components” such as years, months and days, and let the NSDateComponents class give us those as NSIntegers (i.e. long integers). For the class to calculate this correctly we need to put our days into the context of an NSCalendar (gregorian in our case). This is important because different calendars may return different time intervals.

 

Just the Days

If you’re interested in just one particular item (days for example) just leave out any other component on this line:

How about other units?

You can pass as many of those fast enumeration units as you like, separated by pipes. For a full list of available values check out the NSCalendar Class Reference

 

Further Reading