斯威夫特2.0

let today = NSDate()    
let calendar = NSCalendar.currentCalendar()
let tomorrow = calendar.dateByAddingUnit(.Day, value: 1, toDate: today, options: NSCalendarOptions.MatchFirst)

从iOS 8开始，你可以使用NSCalendar.dateByAddingUnit

Swift 1.x中的示例:

let today = NSDate()
let tomorrow = NSCalendar.currentCalendar()
    .dateByAddingUnit(
         .CalendarUnitDay, 
         value: 1, 
         toDate: today, 
         options: NSCalendarOptions(0)
    )

斯威夫特2.0:

let today = NSDate()
let tomorrow = NSCalendar.currentCalendar()
    .dateByAddingUnit(
        .Day, 
        value: 1, 
        toDate: today, 
        options: []
    )

斯威夫特3.0:

let today = Date()
let tomorrow = Calendar.current.date(byAdding: .day, value: 1, to: today)

NSDate *now = [NSDate date];
NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *components = [calendar components:NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit fromDate:now];
NSDate *startDate = [calendar dateFromComponents:components];
NSLog(@"StartDate = %@", startDate);

components.day += 1;
NSDate *endDate = [calendar dateFromComponents:components];
NSLog(@"EndDate = %@", endDate);

斯威夫特4.0

extension Date {
    func add(_ unit: Calendar.Component, value: Int) -> Date? {
        return Calendar.current.date(byAdding: unit, value: value, to: self)
    }
}

使用

date.add(.day, 3)!   // adds 3 days
date.add(.day, -14)!   // subtracts 14 days

注意:如果你不知道为什么代码行以感叹号结尾，可以在谷歌上查找“Swift optional”。

字符串扩展:转换String_Date >日期

extension String{
  func DateConvert(oldFormat:String)->Date{ // format example: yyyy-MM-dd HH:mm:ss 
    let isoDate = self
    let dateFormatter = DateFormatter()
    dateFormatter.locale = Locale(identifier: "en_US_POSIX") // set locale to reliable US_POSIX
    dateFormatter.dateFormat = oldFormat
    return dateFormatter.date(from:isoDate)!
  }
}

日期扩展:转换日期>字符串

extension Date{
 func DateConvert(_ newFormat:String)-> String{
    let formatter = DateFormatter()
    formatter.dateFormat = newFormat
    return formatter.string(from: self)
 }
}

日期扩展:获取+/-日期

extension String{
  func next(day:Int)->Date{
    var dayComponent    = DateComponents()
    dayComponent.day    = day
    let theCalendar     = Calendar.current
    let nextDate        = theCalendar.date(byAdding: dayComponent, to: Date())
    return nextDate!
  }

 func past(day:Int)->Date{
    var pastCount = day
    if(pastCount>0){
        pastCount = day * -1
    }
    var dayComponent    = DateComponents()
    dayComponent.day    = pastCount
    let theCalendar     = Calendar.current
    let nextDate        = theCalendar.date(byAdding: dayComponent, to: Date())
    return nextDate!
 }
}

用法:

let today = Date()
let todayString = "2020-02-02 23:00:00"
let newDate = today.DateConvert("yyyy-MM-dd HH:mm:ss") //2020-02-02 23:00:00
let newToday = todayString.DateConvert(oldFormat: "yyyy-MM-dd HH:mm:ss")//2020-02-02
let newDatePlus = today.next(day: 1)//2020-02-03 23:00:00
let newDateMinus = today.past(day: 1)//2020-02-01 23:00:00

参考:来自多重问题 我如何添加1天到一个NSDate? 数学函数转换正整数为负和负到正? 将NSString转换为NSDate(然后再转换回来)

iOS 8+， OSX 10.9+， Objective-C

NSCalendar *cal = [NSCalendar currentCalendar];    
NSDate *tomorrow = [cal dateByAddingUnit:NSCalendarUnitDay 
                                   value:1 
                                  toDate:[NSDate date] 
                                 options:0];