Every pack of medicine carries an expiry date, and every batch record has to get to that date by a calculation. It looks like simple addition: take the manufacturing date, add the shelf life, and print the result. Plants often disagree about which date counts as the start, whether the last day of the month or the day before is meant, and what to do when a leap year or a short month gets in the way. A one-day difference between two sites can cause a rejected shipment or a recall.
The shelf life itself comes from stability studies and the registered dossier. This article is only about turning it into a correct date.
Shelf life, expiry, retest and beyond-use: four different things
| Term | What it means | Typical use |
|---|---|---|
| Shelf life | The period the product is expected to stay within its specification when stored as labelled. It comes from stability data. | Finished medicines |
| Expiry date | The date after which the product must not be used or supplied. It is shelf life counted from a defined start date. | Finished medicines, printed on the pack |
| Retest date | The date when a material must be tested again before it can be used. It is not an expiry date. | Active ingredients, excipients |
| Beyond-use date (in-use period) | The time a product may be used after it is opened, mixed or reconstituted. | Reconstituted suspensions, multi-dose liquids |
Mixing these terms is the first source of mistakes. For example, a retest date on an excipient certificate does not mean the material has expired, and an in-use period of 28 days after opening is not the printed expiry of the pack.
Step 1:Agree on the start date
Shelf life is counted from a defined starting point, usually called the date of manufacture.Companies and registrations define it in different ways, for example the start of the first blending step, the date a defined critical step was completed, or another date set out in the registered file. The important thing is that your procedure/SOP names one start date, that the same rule is used for every batch of the product, and that it matches what was filed with the authority. Do not use the packing date or the release date unless your registration says so, because that would quietly extend the shelf life.
Step 2: Choose the rule for turning months into a date
Three conventions are common. They can give dates that differ by days or even by a month, so the rule must be written in your SOP.
| Rule | How it works | Mfg 15 Mar 2026, 24 months |
|---|---|---|
| A. Last day of the same month | Add the shelf life to the manufacturing month and use the last day of that month. Printed as month and year. | 31 Mar 2028 (03/2028) |
| B. Day before the same date | Add the shelf life to the exact date, then subtract one day. | 14 Mar 2028 |
| C. Last day of the month before | Add the shelf life, then step back one month and use the last day of that month. Printed as month and year. | 29 Feb 2028 (02/2028) |
When a date is printed as month and year only, it is generally understood to mean the last day of that month. This is the convention in US labelling regulations (21 CFR 201.17) and is widely followed elsewhere, but check the labelling rules of each market you supply.
Worked example 1: one batch, three rules
A batch is manufactured on 15 March 2026 and has a registered shelf life of 24 months.
- Rule A: 24 months after March 2026 is March 2028, and the last day of March is 31 March 2028. Printed: 03/2028.
- Rule B: 15 March 2028 minus one day is 14 March 2028.
- Rule C: one month before March 2028 is February 2028, and its last day is 29 February 2028 because 2028 is a leap year. Printed: 02/2028.
The three results span about a month. That is why a company must pick one rule and apply it everywhere.
As per my experience and observations the rule C is commonly used.
Worked example 2: how much shelf life is left
Customers and distributors often require a minimum remaining shelf life at delivery. Remaining shelf life is the days from the check date to the expiry date, as a share of the total shelf life.
Using rule A, the expiry date is 31 March 2028. The total period from 15 March 2026 to 31 March 2028 is 747 days. On 30 October 2026, 518 days remain, which is 518 ÷ 747 × 100 = 69.3% of the shelf life.
Try it yourself
Quick calculator (reference only)
Use the rule written in your own SOP. The check date shows how much shelf life is left on that day.
Beyond-use dates after opening or reconstitution
Some products have a short use period once they are opened or mixed. The same counting question arises. If a suspension is reconstituted on 30 October 2026 and may be used for 14 days, adding 14 days gives 13 November 2026. Some procedures count the day of preparation as day 1, which gives 12 November. The label wording and your procedure must say which, and the in-use period never extends the printed expiry date of the unopened pack.
Traps to watch for
- Short months. Adding one month to 31 January has no 31 February. Decide whether the result is the end of February or a day later, and apply it consistently.
- Leap years. A date in February can move by one day between years, as the example above shows.
- Different date formats. 03/04/2028 means different things in different countries. Printing the month as a word or using the year-month format avoids confusion I.e 03-APRIL-2028
- Reprocessing and rework. Check whether your registration lets the shelf life start again, or whether the original date continues.
- Packaging changeover. Printed expiry dates are part of the printed material, so the code on the pack must be reconciled and checked. See Packing Reconciliation.
A short checklist before the date goes on the pack
- The registered shelf life and storage conditions have been confirmed for this product and market.
- The start date is the one defined in the procedure, and it is recorded in the batch record.
- The calculation rule is the one in the SOP, and a second person has checked the result.
- The date format matches the market, and the first pack off the line has been checked against the batch record.
- The printing set-up and the reconciliation of printed materials are signed off.
Common mistakes
- Counting from the packing date or release date instead of the defined manufacturing date.
- Using the retest period of a raw material as the shelf life of the finished product.
- Using different rules at different sites for the same product.
- Forgetting that a month-and-year label means the end of the month.
- Ignoring leap years and short months when adding months to a date.
- Treating an in-use period as if it extended the unopened expiry date.
Frequently asked questions
How do you calculate the expiry date of a medicine?
Add the registered shelf life, in months, to the defined date of manufacture, and apply the rule in your procedure for the day, for example the last day of that month.
What does an expiry date of 03/2028 mean?
It is generally understood to mean the last day of March 2028. Check the labelling rules of the market you supply.
Is retest date the same as expiry date?
No. A retest date means the material must be tested again before it can be used. It can then be used for a further period if it still meets its specification.
Can a company extend an expiry date?
Only through approved stability data and the regulatory change process of each market. It is not done by changing the calculation.