The New Year is welcomed in not by the chimes of Big Ben but by the cries of an overly sleepy child. Rose is held snuggly in her father's arms, her face rubbing fretfully against his chest. Luckily she has no recollection of just how horrible the past year has been. John admits the only high point had been the birth of his beloved daughter, from there on in it had been all downhill. Mary's death at the hands of a madman; Sherlock's spiral back into the world of drugs and paranoia brought on by (what he believes to be) his failure to keep his vow; to keep John, Mary and their child safe.
John knows he should feel more over Mary's death but she was little more than a stranger in the end and he has no clue how to mourn a person who never existed. Sherlock, with John's aid and Rose's wet and sticky caresses, has turned himself around and has been clean for four long months. John couldn't be prouder, he knows how hard it has been.
"Happy New Year, John." Sherlock mutters.
"Happy New Year, Sherlock." John returns, shifting the weight of Rose in his arms. Carefully, slowly, he shifts and presses a chaste kiss against the cheek of a shocked Sherlock. "This year will be better."