Chronically ill individual: can you define what the IRS looks for with regard to a doctor's certification of chronic illness, and a doctor's plan of care?

Medical expenses are the costs of diagnosis, cure, mitigation, treatment, or prevention of disease, and the costs for treatments affecting any part or function of the body. These expenses include payments for legal medical services rendered by physicians, surgeons, dentists, and other medical practitioners. They include the costs of equipment, supplies, and diagnostic devices needed for these purposes.

Medical care expenses must be primarily to alleviate or prevent a physical or mental disability or illness. They don't include expenses that are merely beneficial to general health, such as vitamins or a vacation.

Besides deducting the cost of the assisted living facility, you can also deduct part of your life-care fee, if any.  This is from the same publication:

You can include in medical expenses a part of a life-care fee or "founder's fee" you pay either monthly or as a lump sum under an agreement with a retirement home. The part of the payment you include is the amount properly allocable to medical care. The agreement must require that you pay a specific fee as a condition for the home's promise to provide lifetime care that includes medical care. You can use a statement from the retirement home to prove the amount properly allocable to medical care. The statement must be based either on the home's prior experience or on information from a comparable home.

Does it matter that the statement of proof is post dated?  I'm worried regarding the "certified within the 12 month period prior to the care being provided" clause.

Where do you see that clause?

Let me see here... it could only be on the CPA form I found... I know they were being very legalistically specific to attempt to avoid any potential issues.

Pub 502, definition of "chronically ill individual"

Chronically ill individual.
An individual is chronically ill if, within the previous 12 months, a licensed health care practitioner has certified that the individual meets either of the following descriptions.

He or she is unable to perform at least two activities of daily living without substantial assistance from another individual for at least 90 days, due to a loss of functional capacity. Activities of daily living are eating, toileting, transferring, bathing, dressing, and continence.
He or she requires substantial supervision to be protected from threats to health and safety due to severe cognitive impairment.

It's the "within the previous 12 months" that got me worried.

I know his statement will state that the condition existed and the care was necessary in 2017.  Thing is it's going to be dated 2018...

So if the note is dated April 2018 and care was necessary starting April 2017 it'll be all right.