You make a very excellent point and you elucidate well. Well enough to give you an equally thought out answer.
The problem here is you're thinking in a dichotomy; that if I think it's ugly, I think it should have been differently or better. These two do not exclude one another. As a visual device to show passage of time, it's fine. I just think it's, well, off-putting. I dislike the way it looks, but it doesn't have to look nice.
Realistically I can't give you an answer.
The best one I could give would be Stevonnie checking her phone for one reason or another at the beginning, then realizing, "oh, right. No bars out in the middle of an alien planet" with the date in the upper right, clearly visible to the viewer.
And then checking again, just as the battery dies, "I hope this doesn't show up as roaming charges" or something, with the date now several days later. Again, visible to the viewer.
Of course, it would have to be either Steven's or Connie's phone. Or one, then the other.
That's the best alternative I could think of, and it's not very good. But again, just because I think it's ugly, doesn't mean I think it's gotta change.