The expiration date for 1 year /3 devices starts by the moment you activated the first device, which in your situation was 2/26/18. I haven't been able to find the details on this question, so apologies if it is available elsewhere. Is this how it is supposed to work, or should I see the older PC expiration date more like 4/2019? I did that, and while it did accept the key, it shows an expiration of 2/26/19 (one year from the date I put one activation on a different PC) rather than simply adding to the current expiration date.
Yesterday I began getting the message on my existing (expiring 2-year install) PC and I figured I could just change the license key to the new 1-year code to extend the license for another 12 months. At that time (2/26/18) I put one install on a new computer, intending to use the other install on the existing machine.
Knowing that it would be coming up for renewal, I purchased 1-year/3 device package back in February. I have NOD32 installed and running on a 2-year license key from 2016.