How to predict date met eligibility

According to the law you must give 30 days notice before a person becomes eligible to make deferrals to the plan. How is this calculated because a participants hours may vary in any given month? For salaried employees I would think we can make an assumption. The plan doc specifies that the eligibility is 175 hours and is using the actual hours method to calculate eligibility. 

What notice are you referring to in particular? Retirement plan participants are required to receive a number of notices at various times and each of those notices has slightly different timing requirements. While most notices have a latest date on which they may be provided, not all have an earliest date—meaning that you can sometimes just provide the notice when the employee is hired and call it good.

The service requirement is 175 hours over what period of time? A month? 3 months? A year? And what is the entry date once they satisfy the service requirement? You said the plan is using the counting-hours method, does it have an hours equivalency provision? What happens if the participant does not meet the hours requirement during their initial eligibility computation period?

I'm merely speculating that this is a safe harbor plan, and that you are referring to what is commonly called the "30 day notice" requirement. IF that is the case, the law does not necessarily require a notice at least 30 days in advance. The statute requires that the notice be given within a "reasonable" time prior to the beginning of the plan year. For the normal annual notice, the IRS "deems" it to be reasonable if given between 30 and 90 days prior to the beginning of the plan year, but facts and circumstances can override this.

For a newly eligible employee, the requirement is satisfied if the notice is given by the employee's date of eligibility.

You might want to spend some time reading through 401(k)(12) and (13), as well as 1.401(k)-3(d)(3) if your situation is in fact what I'm guessing, as well as IRS Notice 98-52. If your safe harbor is a QACA, see also see 1.401(k)-3(k)(4).