The notice period has nothing to do with the 1st of the month, when the rent is typically due. Either landlord or tenant can give notice in the middle of the month.
If the tenant only provided 15 days' notice when 30 days notice was required, then the tenant still owes 15 days of rent. The landlord can deduct 15 days of rent from the security deposit, provided the landlord gives the tenant an itemized deduction within 21 days after the tenant moves out, as required under Civil Code section 1950.5.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This Avvo.com posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult with your own attorney.