Vacancies not destroying investment? Partial Month's Rent?

[Eric Lilly]

Hey everyone, 

Big Noob here. 21 years old and aggressively saving for my first down payment. Just beginning the process of acquiring resources and learning about rental properties. One thing that stuck out to me was vacancies. Calculating the monthly expenses for a property, even Captial Expenditures, does seem daunting but at least very straightforward. Vacancies however seem like they could siphon away your yearly cash flow so easily. I'm imagining that my first property's monthly rent is $1200, with a mortgage of $550 and total monthly expense cost of $1000. I haven't come across anything so far on "Partial Month's Rent" for lack of a better term. Basically if a tenant moved in on the 12th, do they still pay for the other 18 days of that month and how so? Is it all stated directly in the lease up front? Would it be approximately 60% (18 days / 30 days) of the rent? So would I be able to expect approximately $720 that month? 

Appreciate it,

Eric