I quickly experimented. It's as you say--it seems to pass by # of turns or steps. However, I think traveling on the overworld uses up much more time than towns. Currently, I can exit on the overworld and run around to trigger a new day in a few steps, whereas I can explore a whole town before that happens.
Also, are these the confirmed full list? For example, Krizsan and Shayder had effects too in the originals. I wonder whether I'm on track or not before the first wreckage, since I'm on about day 4 and just hit Ghikra
Going to heal between dungeons is really costly on time