Some useful date extension ideas I have in my mind. Please take a look.

DateExtensions.swift

import Foundation

extension DateFormatter {
    static let instance = DateFormatter()
    // You can also add multiple date formatters, creating date formater is very heavy task, 
    // It is not advisable to create instance of it every time you want to use it.
}
extension Calendar {
    // Shorthands for different calendar
    static let gregorianCalendar = Calendar(identifier: .gregorian)
    static let indianCalendar = Calendar(identifier: .indian)
    static let chineseCalendar = Calendar(identifier: .chinese)
}
extension TimeZone {
    // Shorthands for different timezones
    static let utcTimeZone = TimeZone(identifier: "UTC")
    static let indianTimeZone = TimeZone(identifier: "IST")
}
extension Date {
    func toString(format: String = "EEE, dd MM yyyy") -> String {
        // Crating date formatter instance is very heavy operation, i would recomend to create dateformatter instance only once and use it globally
        DateFormatter.instance.dateStyle = .short
        DateFormatter.instance.dateFormat = format
        return DateFormatter.instance.string(from: self)
    }
    func timeIn24HourFormat() -> String {
        DateFormatter.instance.dateStyle = .none
        DateFormatter.instance.dateFormat = "HH:mm"
        return DateFormatter.instance.string(from: self)
    }
    func firstDateOfMonth() -> Date {
        var components = Calendar.current.dateComponents([.year, .month], from: self)
        components.day = 1
        let firstDateOfMonth = Calendar.current.date(from: components)!
        return firstDateOfMonth
    }
    func lastDateOfMonth() -> Date {
        return Calendar.current.date(byAdding: DateComponents(month: 1, day: -1), to: self.firstDateOfMonth())!
    }
    func nextDate() -> Date {
        let nextDate = Calendar.current.date(byAdding: .day, value: 1, to: self)
        return nextDate ?? Date()
    }
    func previousDate() -> Date {
        let previousDate = Calendar.current.date(byAdding: .day, value: -1, to: self)
        return previousDate ?? Date()
    }
    func addMonths(numberOfMonths: Int) -> Date {
        let result = Calendar.current.date(byAdding: .month, value: numberOfMonths, to: self)
        return result ??  Date()
    }
    func substractMonths(numberOfMonths: Int) -> Date {
        let result = Calendar.current.date(byAdding: .month, value: -numberOfMonths, to: self)
        return result ??  Date()
    }
    func addYears(numberOfYears: Int) -> Date {
        let result = Calendar.current.date(byAdding: .year, value: numberOfYears, to: self)
        return result ??  Date()
    }
    func substractYears(numberOfYears: Int) -> Date {
        let result = Calendar.current.date(byAdding: .year, value: -numberOfYears, to: self)
        return result ??  Date()
    }
}