I have lived in my apartment for three years now, I am on a month to month agreement, recently new landlords took over my apartment building. I never signed a new rental agreement. I want to move out on the 15th of the month, but I was told I have to put my 30 days in at the end of the month, so it is easier for them to move someone else in? I want my rent to be pro-rated, is this possible, for me to leave early and still keep half my rent?

Laws vary from state to state. Where are you?

ANY question about law NEEDS to include your general locale as laws vary. But the new landlords are probably correct. You probably have to give ONE rental period's notice.

In "most" areas, your notice has to be by the rental period, but you can always MOVE, you just have to pay the monthly rental

The answer should be in the most recent lease that you signed. It doesn't matter if it was with the old owners. Providing that it states that the lease will automatically go month-to-month after the initial term was over, then all of the rules for notices, etc, still apply. If the lease doesn't specify that the notice has to cover a rental period, then any 30 day period would suffice, unless that's specified my your state's specific laws.