Determining eligibility when rule of parity and one year holdout both don't exclude any service

If the rule of parity states:
If an Employee does not have any non-forfeitable right to Employer contributions, exclude eligibility service before a period of five (5) consecutive One-Year Breaks in Service/Periods of Severance - No

and the one year holdout state:
If an Employee has a One-Year Break in Service/Period of Severance, exclude eligibility service before such period until the Employee has completed a Year of Eligibility Service after returning to employment with the Employer - No

How far back are we required to look to determine if an employee who joins has any service that should be included because both of these rules do not exclude it? Is there a certain criteria given by the IRS or if it is not determined in the plan document is it like since the company began? 

Is this a 401(k) plan? If so, you need to look back to the employee's records since the company began or back to the effective date of the plan, if later, to see if the employee ever made deferrals. If yes, the employee may become a participant immediately.

The rule of parity only applies to nonvested participants so isn't really relevant in a 401(k) plan.

In general, an employer must keep employment records back to whenever is needed to determine an employee's benefit. See ERISA section 209, which states that an employer must maintain benefit records sufficient to determine the benefits due or that may become due to its employees. So practically speaking, that's back to the beginning of the plan. Of course, real life is another matter, but speaking from experience, benefit claims that come up after an employer has lost, archived, or otherwise no longer has access to employee records, and that go back many years are extremely difficult to deal with. (The ones where the original company doesn't exist anymore or has been bought and sold a few times can be particularly challenging.)