envflag.go
func DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
	EnvironmentFlags.DurationVar(p, name, value, usage)
}
func DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
	EnvironmentFlags.DurationVar(p, name, value, usage)
}
// 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.