envflag.go
func Duration(name string, value time.Duration, usage string) *time.Duration {
	return EnvironmentFlags.Duration(name, value, usage)
}
func Duration(name string, value time.Duration, usage string) *time.Duration {
	return EnvironmentFlags.Duration(name, value, usage)
}
// Duration defines a time.Duration flag with specified name, default
// value, and usage string.  The return value is the address of a
// time.Duration variable that stores the value of the flag.