Swift - check if a timestamp is yesterday, today, tomorrow, or X days ago

Swift 3/4/5:

Calendar.current.isDateInToday(yourDate)
Calendar.current.isDateInYesterday(yourDate)
Calendar.current.isDateInTomorrow(yourDate)

Additionally:

Calendar.current.isDateInWeekend(yourDate)

Note that for some countries weekend may be different than Saturday-Sunday, it depends on the calendar.

You can also use autoupdatingCurrent instead of current calendar, which will track user updates. You use it the same way:

Calendar.autoupdatingCurrent.isDateInToday(yourDate)

Calendar is a type alias for the NSCalendar.

Swift 4 update:

let calendar = Calendar.current
let date = Date()
        
calendar.isDateInYesterday(date)
calendar.isDateInToday(date)
calendar.isDateInTomorrow(date)

Calendar has methods for all three cases

func isDateInYesterday(_ date: Date) -> Bool
func isDateInToday(_ date: Date) -> Bool
func isDateInTomorrow(_ date: Date) -> Bool

To calculate the days earlier than yesterday use

func dateComponents(_ components: Set<Calendar.Component>, 
                      from start: Date, 
                          to end: Date) -> DateComponents

pass [.day] to components and get the day property from the result.

---

This is a function which considers also is in for earlier and later dates by stripping the time part (Swift 3+).

func dayDifference(from interval : TimeInterval) -> String
{
    let calendar = Calendar.current
    let date = Date(timeIntervalSince1970: interval)
    if calendar.isDateInYesterday(date) { return "Yesterday" }
    else if calendar.isDateInToday(date) { return "Today" }
    else if calendar.isDateInTomorrow(date) { return "Tomorrow" }
    else {
        let startOfNow = calendar.startOfDay(for: Date())
        let startOfTimeStamp = calendar.startOfDay(for: date)
        let components = calendar.dateComponents([.day], from: startOfNow, to: startOfTimeStamp)
        let day = components.day!
        if day < 1 { return "\(-day) days ago" }
        else { return "In \(day) days" }
    }
}

Alternatively you could use DateFormatter for Yesterday, Today and Tomorrow to get localized strings for free

func dayDifference(from interval : TimeInterval) -> String
{
    let calendar = Calendar.current
    let date = Date(timeIntervalSince1970: interval)
    let startOfNow = calendar.startOfDay(for: Date())
    let startOfTimeStamp = calendar.startOfDay(for: date)
    let components = calendar.dateComponents([.day], from: startOfNow, to: startOfTimeStamp)
    let day = components.day!
    if abs(day) < 2 {
        let formatter = DateFormatter()
        formatter.dateStyle = .short
        formatter.timeStyle = .none
        formatter.doesRelativeDateFormatting = true
        return formatter.string(from: date)
    } else if day > 1 {
        return "In \(day) days"
    } else {
        return "\(-day) days ago"
    }
}

Update:

In macOS 10.15 / iOS 13 RelativeDateTimeFormatter was introduced to return (localized) strings relative to a specific date.

NSCalender has new methods that you can use directly.

NSCalendar.currentCalendar().isDateInTomorrow(NSDate())//Replace NSDate() with your date
NSCalendar.currentCalendar().isDateInYesterday()
NSCalendar.currentCalendar().isDateInTomorrow()

Hope this helps