Part One: On The Beach.
Chapter 1, The Tree House, ends as Dolph leaves Earth.
Chapter 2, The Sea of Streams, ends as Dolph approaches Mars.
Chapter 3, Down...and Out, ends with Dolph marooned on Mars.
Chapter 4, A Little Detection, Dolph's inventory; Nanette's deduction.
Chapter 5, Morning on Mars, first day on Mars.
Part Two: Friday.
Chapter 6, Evening on Mars, Nanette arrives.
Chapter 7, Children in the Sky, twenty-nine days after Dolph left.
Chapter 8, Wine and Storm, Nanette has just arrived but Dolph has been on Mars for weeks (p. 80)!
"Nanette had noticed that the tree-house was missing, on the very morning of its departure." (p. 46)
Before following Dolph to Mars, she leaves a note for her and his parents, saying that he has not gone camping but has gone to Mars and that she has followed him. She must have left the note, before leaving Earth, shortly after realising that he had gone and within the five or so days when Dolph was believed to be camping.
The novel describes, in this order:
Dolph seeing Nanette's arrival on Mars (end of Chapter 6);
the "Children in the Sky" news story on Earth, ending twenty nine days after Dolph left Earth (Chapter 7);
Dolph rescuing the unconscious Nanette, with the auctorial comment, "For him, it was very clear, the case of the Children in the Sky was very far from over." (p. 77) (end of Chapter 7);
Nanette, the day after her arrival, finding "...the root of the irregularity in the pump cycle, which had been eluding him for weeks." (p. 80) (early in Chapter 8)
Has this chronology of fictitious events become inconsistent? I wish that I had read the text this closely while Blish was still alive.
Later: I did not address above the question of how long it takes Dolph and Nanette to travel to Mars. But the implication is that it is not long. Dolph intended to be away for about five days and:
"By the middle of his second day in space he was already slightly more than half-way there..." (p. 28)
Nanette should not have taken much longer and, in any case, could not have survived for weeks in space.