How does one subtract hours from an NSDate?

Question

NSDate *newDate = [theDate dateByAddingTimeInterval:-3600*4];

Link to documentation.

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NSDate *newDate = [[[NSDate alloc] initWithTimeInterval:-3600*4
                                              sinceDate:theDate]] autorelease];

Link to documentation.

NSCalendar is the general API for changing dates based on human time units. For this, you can use NSCalendar's -dateByAddingComponents:toDate:options: with a negative number of hours.

In Swift 4 :

var baseDate = ... // something
let dateMinus4Hours = Calendar.current.date(byAdding: .hour, value: -4, to: baseDate)

don't go with 24*3600 and stuff, that's asking for trouble.

Since iOS 8 there is the more convenient dateByAddingUnit:

Swift 2.x

//subtract 3 hours
let calendar = NSCalendar.autoupdatingCurrentCalendar()
newDate = calendar.dateByAddingUnit(.Hour, value: -3, toDate: originalDate, options: [])

//in Swift 3
//subtract 3 hours
let calendar = NSCalendar.autoupdatingCurrent
newDate = calendar.date(byAdding:.hour, value: -3, to: originalDate)

Here a function which might be useful as it returns the date -4 h considering that this may also change the date and the month and eventually the year. the .searchBackward option is the important part :)

public static func correctSecondComponent(date: Date, calendar: Calendar = Calendar(identifier: Calendar.Identifier.gregorian))->Date {

    let hour = calendar.component(.hour, from: date)

    let e = (calendar as NSCalendar).date(byAdding: NSCalendar.Unit.hour, value: -4, to: date, options:.searchBackwards)!

    return e
}

dateFromString function returns NSDate, not NSString. you should change,

NSDate * theDate = [dateFormatter dateFromString:datetemp];