My dad’s shuttered small business was audited last century. In his case they sent a letter saying you are under an investigation for year x, they clock resets

My dad’s shuttered small business was audited last century. In his case they sent a letter saying you are under an investigation for year x, they clock resets to x years after that and they can bring action anytime. At least that’s the was it was explained to very young me at the time.