How to get the today's and tomorrow's date in swift 4

This can be done very simply using the following code. No need for date components or other complications.

var calendar = Calendar.current
// Use the following line if you want midnight UTC instead of local time
//calendar.timeZone = TimeZone(secondsFromGMT: 0)
let today = Date()
let midnight = calendar.startOfDay(for: today)
let tomorrow = calendar.date(byAdding: .day, value: 1, to: midnight)!

let midnightEpoch = midnight.timeIntervalSince1970
let tomorrowEpoch = tomorrow.timeIntervalSince1970

I would do this with components.

Assuming you need time in seconds as defined by time(2). If you need in milliseconds as defined by time(3), then you can multiply it out by 1000.

// Get right now as it's `DateComponents`.
let now = Calendar.current.dateComponents(in: .current, from: Date())

// Create the start of the day in `DateComponents` by leaving off the time.
let today = DateComponents(year: now.year, month: now.month, day: now.day)
let dateToday = Calendar.current.date(from: today)!
print(dateToday.timeIntervalSince1970)

// Add 1 to the day to get tomorrow.
// Don't worry about month and year wraps, the API handles that.
let tomorrow = DateComponents(year: now.year, month: now.month, day: now.day! + 1)
let dateTomorrow = Calendar.current.date(from: tomorrow)!
print(dateTomorrow.timeIntervalSince1970)

You can get yesterday by subtracting 1.

If you need this in the universal time (UTC, GMT, Z… whatever name you give universal time), then use the following.

let utc = TimeZone(abbreviation: "UTC")!
let now = Calendar.current.dateComponents(in: utc, from: Date())

Use this extension to get today's and tomorrow's date

extension Date {
   static var tomorrow:  Date { return Date().dayAfter }
   static var today: Date {return Date()}
   var dayAfter: Date {
      return Calendar.current.date(byAdding: .day, value: 1, to: Date())!
   }
}

Also try adding following code in date extension:

extension Date
{
    var startOfDay: Date 
    {
        return Calendar.current.startOfDay(for: self)
    }

    func getDate(dayDifference: Int) -> Date {
        var components = DateComponents()
        components.day = dayDifference
        return Calendar.current.date(byAdding: components, to:startOfDay)!
    }
}

You can use the following method to get any date by adding days or months or years by specifying the Calendar Component and the increment value of this component:

func getSpecificDate(byAdding component: Calendar.Component, value: Int) -> Date {

    let noon = Calendar.current.date(bySettingHour: 12, minute: 0, second: 0, of: self)!

    return Calendar.current.date(byAdding: component, value: value, to: noon)!
}

Where the component wil be one from the following option : ( .day , .month , .year ) and the value will be the amount you want to add for this component

for example to get the next year date you can use the following code:

var nextYear = getSpecificDate(byAdding: .year, value: 1).timeIntervalSince1970