If I were to guess, I'd say the Nissan server holds the command, and sends it to the car at the specified time. I'm guessing that because if I send a remote command to immediately charge, I get a text message back that says "charging started." If I set the timer for 5AM, then I get the exact same text message at 5AM, so I'm assuming the car doesn't know the difference. Either way it gets a "charge now" message. When charging starts because of the in-car timer, I don't recall getting any kind of message.
But of course, I am guessing.
I like the feature, though, because I'm able to set a 24 hour timer to charge to 80%. Then, if I'm lying in bed and I decide I'm going to leave the house at 8:30AM, I just send a remote charge command to start at 6:30, since a 80%->100% charge takes about 2 hours. In this way, the car is always "ready enough" (80%) to go at any moment, but always at 100% in the morning for a full day of whatever comes. The once or twice a month that I've come home with less than 20%, I've really appreciated being able to do this.