I've heard some people claim these are the same, and others saying they're different. What is correct?
Is the employee working on Aug15? I am unclear why the two pieces of information would be different.
Once upon a time I was part of a large HRIS installation and the fields were all user defined. We basically sat down and determined WHY we needed each piece of information and if there really was a difference between the two items you mentioned we would have a StartWork field and a FirstDayPaid field. We would not try to make one field do two unrelated things. I do not remember every seeing that distinction although we did have a StartWork and StartBenefits field. And three different fields associated with termination of employment.
With most companies StartDate = FirstDayWorked. That is why when you used defined fields you need to "define" exactly what the field means. There are legal reasons to track FirstDayWorked. Now you are talking about BenefitStartDate, which is normally a function of the benefit plans, and which is normally not the same as FirstDayWorked. Hence the reason for not having one field which has do two unrelated things. For most companies I have worked for BenefitStartDate = FirstFullMonthWorked, but different companies do different things.
Check your benefit plan and see what YOUR company's rules are for vacation. That is not normally ERISA, so it should be at the company policy level. Although almost any plan can become ERISA if funded, some plans are inherently ERISA. All ERISA plans must have a formal plan structure (not company policy) and are generally funded through an outside trustee. Vacation is normally not-ERISA, not funded, no formal plan, just a company policy.
Meaning you go on vacation whenever your company says you do. I have never heard of any vacation going live 30 days before work started, but it is not impossible, just improbable.