Two years of operating system upgrades (Android - one) - how is it implemented? "Android One phones will receive at least two years of OS upgrades," according to Google. 

I interpret this statement as follows.


  An Android One phone will receive at least two major version upgrades for Android within two years of the device's launch date. For example, an Android One device that launched with Android 8.0 will eventually receive upgrades for versions 9.0 and 10.0.


Is this interpretation correct? Am I missing details?

To clarify, Android 8.0 was a major release; Android 8.1 was a minor maintenance release.
android-one phones are produced by multiple OEMs, so they are not governed by a fixed date of release. But they are mandated to provide two years of OS upgrades and three years of security updates. To remove the confusion of start date and end date, which is your question, see the fine print at the bottom of page you linked which says, OEMs should



  "Confirm exact duration of support for phones in your territory with smartphone manufacturer"


This applies both to OS and monthly updates as you can s from Android One Wikipedia 

So, you need to check the OEM support for the region to be sure. However, since they are monitored by Google, you can be sure you will get updates as promised. 


The general trend is that they release devices into the market before/after the next major update, for example, if Android 8 is to be released in a few months, the phones out of the box come with Android 7 and are supported till update to Android 9. That said, how soon they release the next version is a big variable. Your best bet, would be to follow Counterpoint comparison or technical reviews /XDA


Edit: Post your edit to your question, yes, your interpretation is correct.