[add] Add function getDateString
[add] Add function getDateString

about getDateString
  in js
  string getDateString(Date _date, string _format)
  initial value
    _date: new Date()
    _format: '%Y/%m/%d %H:%M:%S'
  arguments
    _date
      Date object
    _format
      %Y: 4 digits year
      %y: 2 digits year
      %m: 2 digits month
      %d: 2 digits day
      %H: 2 digits hour
      %M: 2 digits minute
      %S: 2 digits second

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