remind birthday one month

remindbirthday.swift

func checkDates(birthdays: [Date], today: Date, days: Int) -> Set<Date> {
    let calendar = Calendar.current
    let thisYear = calendar.component(.year, from: today)
    var celebrateList = Set<Date>()
    var datesToCheck: ClosedRange<Date> {
        return today...today.addingTimeInterval(3600*24*Double(days))
    }
    
    for birthday in birthdays {
        var dateCompenents = calendar.dateComponents([.year, .month, .day], from: birthday)
        dateCompenents.year = thisYear
        let birthdayOfThisYear = calendar.date(from: dateCompenents)
        if let birthdayOfThisYear = birthdayOfThisYear, datesToCheck.contains(birthdayOfThisYear) {
            celebrateList.insert(birthday)
        }
    }
    
    return celebrateList
}

1. change datesToCheck from computed variable to immutable stored variable.
2. check days to ensure it to be equal of greater than 0.