print(1)
// 結果 -> 1
print(String(format: "%02d", 1))
// 結果 -> 01
例:
var year = 2015
var month = 1
var day = 31
var hour = 1
var min = 6
print("今の年月日日時は \(String(format: "%04d", year))/\(String(format: "%02d", month))/\(String(format: "%02d", day)) \(String(format: "%02d", hour)):\(String(format: "%02d", min))")
// 結果 -> 今の年月日日時は 2015/01/31 01:06