How do I create an NSDate object with a custom date other than the current date? For example I would like to create a var of yesterday or of 2 days ago.
Answer
You should use NSCalendar for calculating dates. For example, in Swift 3 the date two days before today is:
let calendar = Calendar.current
let twoDaysAgo = calendar.date(byAdding: .day, value: -2, to: Date())
Or in Swift 2:
let calendar = NSCalendar.currentCalendar()
let twoDaysAgo = calendar.dateByAddingUnit(.Day, value: -2, toDate: NSDate(), options: [])
Or to get the first of the month, you can get the day, month and year from the current date, adjust the day to the first of the month, and then create a new date object. In Swift 3:
var components = calendar.dateComponents([.year, .month, .day], from: Date())
components.day = 1
let firstOfMonth = calendar.date(from: components)]
Or in Swift 2:
let components = calendar.components([.Year, .Month, .Day], fromDate: NSDate())
components.day = 1
let firstOfMonth = calendar.dateFromComponents(components)
There are lots of useful functions in the NSCalendar/Calendar class, so you should investigate that further. See the NSCalendar class reference for more information.
But I would advise against doing any manual adjustments of date objects by adjusting it by some time interval that is a multiple of the seconds per day (e.g. 24*60*60). That technique works fine if you're just adding some time interval, but for date calculations, you really want to use calendar object, to avoid problems stemming from daylight savings and the like.