Hours of Service for Eligibility

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A client would like to have a new plan set up with an eligibility requirement of 6 months and 500 hours of service. I was previously told that if a plan requires less than 12 months of service, an hours requirement cannot be used. I've also seen volume submitter documents that allow you to specify a number of hours required for each month during the eligibility period (seemingly even if the period was less than 12 months long). Is not having an hours requirement for less than 12 months of service a safe-harbor rule or can you have an hours requirement no matter what the circumstances are and no matter how short the eligibility period is? All help is appreciated.