Have you looked at your lease to determine whether there is a provision regarding this scenario? Usually, it will control as to how late payments are determined. For instance, is there language in your lease resembling something like "if payment is not received by xxx days after due date, then a late fee is due and payable"?
If so, then your rent must be received by the owner/landlord by that date, regardless of the method you used.
DISCLAIMER This answer is provided solely for informational purposes only. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising.
The key word in the prior poster's answer is "received." And, when an item is "received" oftentimes depends on what the lease specifies (ometimes it's "received" when the landlord has it, sometimes it's "received" when it's in the mailbox, etc.) and what is the usual and customary method of rent delivery (that is, do you mail your rent every month, etc., or did you just place it in the landlord's mailbox...)
Unfortunately for you, the real answer to your question will require that an attorney sit down with you and go through your lease and ask you some specific questions - it just cannot be answered here given the information you have provided.
So please consult with an attorney directly on this. If you are of limited means, you might try contacting Statewide Legal Services (dial 211 for their number).