This is a room rental situation.

I make very clear to my tenants that 30 days notice of intent to leave is very important to me and I also make clear that I take the first and last month up front so that I can withhold the difference to make up for it if they do not give me their notice. This tenant only gave me 18 days. It distinctly states in my lease that I am allowed to do this. So... is that going to be a problem?

40.Notice of Intent to Vacate. Lessee shall advise Lessor of any changes in terms of tenancy with advance notice of at least 30 days. Changes may include any reasonable changes allowed in the terms of this Agreement. If proper notice is not given, Lessor retains the right to hold Lessee’s last month’s rent, which was paid upfront at the time of this lease.

Lessor shall advise Lessee of any changes in terms of tenancy with advance notice of at least 30 days. Changes may include notices of termination, rent adjustments or other reasonable changes in the terms of this Agreement.

Of course, I'm not talking about keeping the whole thing... just to the end of the 30 days... so divide the last month upfront by 30 and keep 12 days worth of it for the 12 days that I wasn't notified of.

Gaps between tenants are a big problem for me. I rent rooms to make ends meet and usually I have two tenants but currently I have one leaving (this one) and one who isn't paying me, who I just served a notice to...

@Sophie Spatafore  

I don't see an issue with it unless it violate local law. I have the same rule and do the same thing! I had a tenant break the lease and only give me 30 days not 60 days notice. As stated in the lease the difference was a fee and they paid it.