19 is the way to go I suppose!
Okay, so let me try and break this down for you. Of course I have limited knowledge so more experienced members can correct me if I'm wrong.
If I understand correctly, you want to apply for the renewal in July (for example) with a new CoS having a start date of 1 day after the current leave is set to expire. Here's the problem with that:
When you were initially issued Tier 2 visa, the start date on CoS refers to the date of beginning of employment. So if the CoS was assigned on 1st September, for example, it had to be used within 90 days of issuance to apply for the Visa. The start date for Visa then being December 1st. CoS now remains assigned to you until the end of current visa's expiry.
Now, with renewal, you cannot use an already assigned CoS to apply for a renewal of the current visa. Your employer therefore assigns a new CoS to you. Once the new CoS is issued, you have to use it within 90 days to apply for the renewal. Also, and I should have mentioned this before, the start date in your new CoS cannot be more than 3 months from the date of the application.
Once the decision is made on your renewal application, the new visa will be valid from the date the BRP is issued (and not from 1 Day + the current visa expiry) until 14 days + the 'Until' date mentioned on your new CoS.
Your new visa (BRP card actually) will have the new CoS number mentioned on it as well.
Hope this helps.