Relative Time

RelativeTime.swift

extension NSDate {
    var relativeTime: String {
        
        let calendar = NSCalendar.currentCalendar()
        let components = calendar.components([.Second, .Minute, .Hour, .Day, .Month, .Year], fromDate: self, toDate: NSDate(), options: [])
        
        if components.year > 0 {
            if components.year < 2 {
                return "Last year"
            } else {
                return "\(components.year) years ago"
            }
        }
        
        if components.month > 0 {
            if components.month < 2 {
                return "Last month"
            } else {
                return "\(components.month) months ago"
            }
        }
        
        if components.day >= 7 {
            let week = components.day/7
            if week < 2 {
                return "Last week"
            } else {
                return "\(week) weeks ago"
            }
        }
        
        if components.day > 0 {
            if components.day < 2 {
                return "Yesterday"
            } else  {
                return "\(components.day) days ago"
            }
        }
        
        if components.hour > 0 {
            if components.hour < 2 {
                return "An hour ago"
            } else  {
                return "\(components.hour) hours ago"
            }
        }
        
        if components.minute > 0 {
            if components.minute < 2 {
                return "A minute ago"
            } else {
                return "\(components.minute) minutes ago"
            }
        }
        
        if components.second > 0 {
            if components.second < 5 {
                return "Just now"
            } else {
                return "\(components.second) seconds ago"
            }
        }
        
        return ""
    }
}

Swift 3:

extension Date {
    var relativeTime: String {

        let calendar = Calendar.current
        let components = calendar.dateComponents([.second, .minute, .hour, .day, .month, .year], from: self, to: Date())

        if let years = components.year, years > 0 {
            return  years == 1 ? "Last year" : "\(years) years ago"
        }
        else if let months = components.month, months > 0 {
            return months == 1 ? "Last month" : "\(months) months ago"
        }
        else if let days = components.day, days > 0 {
            let weeks = days / 7
            if weeks > 0 { return weeks == 1 ? "Last week" : "\(weeks) weeks ago" }
            else { return days == 1 ? "Yesterday" : "\(days) days ago" }
        }
        else if let hours = components.hour, hours > 0 {
            return hours == 1 ? "An hour ago" : "\(hours) hours ago"
        }
        else if let minutes = components.minute, minutes > 0 {
            return minutes == 1 ? "A minute ago" : "\(minutes) minutes ago"
        }
        else if let seconds = components.second, seconds >= 5 {
            return "\(seconds) seconds ago"
        }
        else { return "Just now" }
    }
}