Call me naïve, but I’ve always assumed that if a government document says it expires on a certain date, it will be valid until that date has passed. When it comes to passports, however, the “best before” date isn’t necessarily the expiry date you see on the document.

I learned this lesson on a recent trip to Northern Ireland. I was allowed into the country, by a security guard mentioned the expiry date as I was leaving. Since nothing was mentioned on my arrival, I assumed he must have his facts wrong. But the comment was strange enough that I felt the need to check with the Canadian passport office when I got home.

Yup, it’s true.

Why is this happening? Several reasons have been given depending on who you talk to. Some say it’s an effort to ensure travelers won’t be caught short if they are hospitalized while out of the country or have some other accident that prolongs their stay. Others suggest it is nothing more than a cash grab by the passport-issuing country. But it isn’t the issuing country that is making the rules.

The woman I spoke with at the Canadian passport office offered this advice: Not only should you ensure that there is enough time remaining for you to complete your trip, but if it is even close to expiring there may be problems. Some countries, such as Thailand and Australia, require a minimum of six months to be remaining on the passport after the date you plan to leave their country. Italy is a bit more forgiving as it only requires three months of remaining time. The length of time varies from country to country.

So think of scrutinizing your passport as an ounce of prevention, much as you would consider what shots might be required when traveling to different parts of the world. In either situation, if you don’t take proper precautions before you leave home, you might have a nasty experience when abroad.

For more information on time limitations, contact the embassy or trade consulate of the country you plan to visit, or contact your own country’s passport office.