I. Official methods

DateComponents

A date or time specified in terms of units (such as year, month, day, hour, and minute) to be evaluated in a calendar system and time zone.

The date or time specified in units to be calculated in the calendar system and time zone, such as years, months, days, hours, and minutes.

2. Implement

1. Calculate the difference between two string dates

func dateDiff(a) -> Int {
  // Calculate the difference between two dates and return the difference of days
  let formatter = DateFormatter(a)let calendar = Calendar.current
  formatter.dateFormat = "yyyy-MM-dd"
  let today = Date(a)// Start date
  let startDate = formatter.date(from: "2021-06-08")
  
  // End date
  let endDate = formatter.date(from: "2021-06-09")
  let diff:DateComponents = calendar.dateComponents([.day], from: startDate!, to: endDate!)
  return diff.day!
}
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2. Calculate the difference between the day and the day

func checkDiff(a) -> Int {
  // Calculate the difference between two dates and return the difference of days
  let formatter = DateFormatter(a)let calendar = Calendar.current
  formatter.dateFormat = "yyyy-MM-dd"

  / / in the day
  let today = Date(a)let startDate = formatter.date(from: formatter.string(from: today))
  
  // Fixed date
  let endDate = formatter.date(from: "2021-06-09")
  
  let diff:DateComponents = calendar.dateComponents([.day], from: startDate!, to: endDate!)
  return diff.day!
}
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Blog links

Swift calculates the difference between the two dates