code: func (fs *FlagSet) DurationVar(p *time.Duration, names []string, value time.Duration, usage string) {
  	fs.Var(newDurationValue(value, p), names, usage)
  }
docs: // DurationVar defines a time.Duration flag with specified name, default value, and usage string.
  // The argument p points to a time.Duration variable in which to store the value of the flag.