I was just thinking about that, funny enough!
That would seem to be a logical explanation, Darkououmon, but "hatch-day" has been used in a context to imply a birthday (Cera says something like "it feels like my hatch-day...it never seems to get any closer")...so maybe they can't make up their minds? I mean, they use the words both "dream" and "sleepstory", "rain" and "skywater", "rainbow face" and "sky colors", and so on.