I see it's been a while since you asked this... here's my answer in case you are still wondering...
I asked the exact same question a few months ago
. I was advised that they really do mean all the time your partner was in the US and all of the time you were out of the US. I also heard that they're lenient if you can't get all the info. I'm not sure what that means.... whether they have some kind of guidelines or if it was just based on people's experiences.
If it helps, up until a few years ago they stamped US passports at the border, so I could figure out most of the time I spent away from the US from that. For the rest I had to dig out old flight itineraries from my email.