Swift 日期-获取距离当前日期，几年、几月、几日的日期

/// 获取距离当前日期，几年几月几日的日期
///
/// - Parameters:
///   - year: 距离当前日期的前/后几年  如：去年:-1   明年:1  今年:0
///   - month:  距离当前日期的前/后几个月  如：上个月:-1   下个月:1  这个月:0
///   - day:  距离当前日期的前/后几天 如：昨天:-1   明天:1   今天:0
/// - Returns: 返回所要的日期
func CurrentDateToWantDate(year:Int,month:Int,day:Int)->Date {
    let current = Date()
    let calendar = Calendar(identifier: .gregorian)
    var comps:DateComponents?
    
    comps = calendar.dateComponents([.year,.month,.day], from: current)
    comps?.year = year
    comps?.month = month
    comps?.day = day
    return calendar.date(byAdding: comps!, to: current) ?? Date()
}
//三个月后的日期 
let resultDate = CurrentDateToWantDate(year:0,month:3,day:0)
//当前日期
print(Date())
print(resultDate)
let calendar = Calendar(identifier: .gregorian)
//中间一共有多少天
let diff:DateComponents = calendar.dateComponents([.day], from: Date(), to: resultDate)
print(diff.day as Any)