The problem is that he was born one week earlier than the blanket says.
The blanket says he was born one day, but it's not true.
I would suggest that Miss Tuppence should go to bed, and that you and I, Mr. Hersheimmer, should share the vigil." Tuppence was about to protest, but happening to glance at the bed she saw Mrs. Vandemeyer, her eyes half-open, with such an expression of mingled fear and malevolence on her face that it quite froze the words on her lips.