I gave notice 8/4/15 that I planned to be ready to hand over the keys 9/6/15. My lease does not specify a notice period or address prorating the last months rent. In fact, it's just a one year lease and it doesn't even say that it goes month to month after that. I've been here 4 years and just kept paying the same amount each month without signing a new lease. Can I prorate my last month's rent? I am in Missouri if it matters.
After the expiration of the original one-year lease but before the landlord accepted another payment of rent, you no longer had any right to remain on the property and you were a "tenant at sufferance", liable to be evicted at any time (subject to due process of law). However, by accepting rent after the end of the lease, the landlord implicitly consented to your continued presence on the property (so you were no longer a tenant at sufferance) and the two of you thereby entered into a new, unwritten, rental agreement.
White v. Marshall, 83 SW3d 57 (Mo Ct App 2002).
In Missouri, unwritten rental agreements "shall be held and taken to be tenancies from month to month", subject to various exceptions that are probably not relevant here. §
441.060.3, RSMo 2014.
The statute goes on to say a month-to-month tenancy "may" be terminated "by a written notice given to the other party stating that the tenancy shall terminate upon a periodic rent-paying date not less than one month after the receipt of the notice". § 441.060.4(1), RSMo 2014. The statute does not specifically say that that is the only way to terminate a tenancy, but "[c]ases have interpreted these requirements to mean that the tenancy can
only be terminated at the end of a rental period and notice of that termination must be given at least one rental period prior to the termination date".
Davidson v. Kenney, 971 SW2d 896, 898 (Mo Ct App 1998) (emphasis mine).
From the OP, it sounds like you may not have complied with these requirements. In particular, if you pay rent on the first of each month, and if we assume the landlord received your notice on August 4, 2015, it would appear the earliest the tenancy could effectively end is October 1, 2015, and you would be liable for rent through to that date. Since the statute makes the relevant date when the notice is
received by the landlord, you would be well advised to send the notice through some method that allows you to prove receipt, such as certified mail.
This is general information only and there may be facts relevant to your situation that are not disclosed in the OP. The intent here is to point you in the right direction for your research, but you shouldn't rely on this post without retaining counsel for a full appraisal of all the details.