How a pharmacist (or your insurance company for that matter) counts a 'day supply' for a prescription can be somewhat confusing.
Complicating matters, there is no state law that provides guidance on whether or not to count the day you pick up the prescription or to start on the next day.
Additionally, in my experience at least, many pharmacists do not do a good job of explaining to patients how exactly they are counting your days.
I am a pharmacist in New York, and the controlled substance law (Part 80) only specifies that a controlled substance prescription cannot be filled more than 7 days early.
Per Part 80 (New York Controlled Substance Law):
"No additional prescriptions for a controlled substance may be issued by a practitioner to an ultimate user within 30 days of the date of any prescription previously issued unless and until the ultimate user has exhausted all but a seven days' supply of that controlled substance provided by any previously issued prescription."
Regardless of what the laws are in your state, many pharmacies have their own, stricter, policy on how many days early you are allowed to fill your controlled substance prescription, which is what is happening in your situation.
How Do Pharmacists Count Day Supply?
In terms of how pharmacists count the 'day supply' of your prescription, standard practice is that the first day you pick up your prescription is not counted, although this is somewhat misleading because the day you finish on is not the day you are due.
This is a very key point.
The day you are 'due' for a refill, you will be out of medication.
To put it another way, you are not due for a refill on the day you run out of medication, you are due the day after (which would be the day you would need your next dose).
Let's use a 10-day antibiotic prescription example to illustrate this.
Day Supply Example One
You pick up a 10-day antibiotic course on February 19, 2019. You want to know when you would finish the prescription, and when you would be due for a refill, (if you needed one).
Assuming you started on February 19, 2019, you would run out of your 10-day course on February 28, 2019. However, you technically are due for a refill on March 1st, 2019. This is how most pharmacists and nearly all insurance companies calculate day supply.
Day Supply Example Two
Now, let's use your fentanyl example.
You picked up a 30-day supply of fentanyl on February 19, 2019.
Based on how your pharmacist (and insurance company) will calculate the day supply, you will be due for a refill on March 21st, which is 30 days. Per the policy at your pharmacy, 2 days early would, therefore, be March 19th.
Remember, since you are started using patches on February 19, 2019, you will finish your last dose on March 20th. However, as stated, you are due on March 21st.
This method of counting day supply is fairly standard, but whether or not it is in the best interest of the patient is certainly questionable.
A conversation for another article is how fair it is to hold a patient, who may have other obligations and can't make it to a pharmacy, to a pick-up date in which they will be out of medication.
Not counting the first day you pick up a prescription in your 'day supply' count isn't technically wrong, but is certainly restrictive.
Instead of thinking of your prescription being counted in terms of the day, think about it in terms of 24-hour intervals, and it makes a little more sense.
Going back to your example, you pick-up a prescription on February 19th. Let's say, at 1 PM.
Twenty-four hours (one day) from February 19th, at 1 PM, is February 20th at 1 PM. That is why the first day "isn't counted", but it technically is...it's just that an elapsed time of 'one day' goes into the next.
Answer SummaryIn general, the day you pick up your controlled substance prescription is not counted as 'day one' when calculating your 'day supply'. Instead, the next day is considered 'one day'.
For example, picking up a prescription on March 1st, March 2nd would be 'day one', not 'day two'.
Using this (standard) calculation for days supply means that you will be out of medication on 'day thirty'.