Check out example codes for "get day start time in swift". It will help you in understanding the concepts better.

Code Example 1

extension Date {

    var startOfDay : Date {
        let calendar = Calendar.current
        let unitFlags = Set<Calendar.Component>([.year, .month, .day])
        let components = calendar.dateComponents(unitFlags, from: self)
        return calendar.date(from: components)!
   }

    var endOfDay : Date {
        var components = DateComponents()
        components.day = 1
        let date = Calendar.current.date(byAdding: components, to: self.startOfDay)
        return (date?.addingTimeInterval(-1))!
    }
}
//Usage:

    let myDate = Date()
    let startOfDate = myDate.startOfDay
    let endOfDate = myDate.endOfDay

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