What is rent for a part month?
A tenant who moves in on the 18th does not owe a month's rent, and one who leaves on the 9th does not either. This works out what they do owe, on whichever basis your lease uses.
There is no jurisdiction picker on this page, and that is not an omission. No California statute prescribes a method for prorating rent. Not the state, not LA City, not any of the rent-stabilised cities. Whichever basis your lease names is the one that governs you. Where the lease says nothing, the actual days in the month is the basis that never charges for a day that does not exist.
How do I work out rent for part of a month?
Prorated rent is the share of a month’s rent that covers only the days a tenant actually has the place, such as a move-in partway through the month or a move-out before it ends. There are two ways to split it: divide the rent by the days that month really has, or divide it by a fixed month length used for every month of the year. No California statute picks one, so the lease is what decides, and the two do not give the same answer. This is plain arithmetic rather than a local rule, so it holds anywhere in California. The tool shows the figure on the basis you choose and what the other basis would have said, so you can see the gap before you put a number in writing.