code: func (f *FlagSet) DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
  	f.Var(newDurationValue(value, p), name, 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.
  // The flag accepts a value acceptable to time.ParseDuration.