Both conventions are legitimate, and both appear in the literature. The Britannica entry notes that the rescue was “about 2.5 months” after the crash. The Wikipedia entry and most popular accounts use 72 days. Parrado’s memoir and his public appearances consistently use 72 days. For purposes of this account, 72 days is the accurate description of the survival period as it is conventionally counted and as the survivors themselves describe it.
The survivors spent those 72 days:
Building and maintaining shelter in the fuselage of the crashed aircraft, which provided minimal but essential protection against conditions that would have been rapidly fatal without it.
Finding and managing water, primarily by melting snow in aluminum trays made from aircraft debris.
Managing a food supply that ran out within the first two weeks and was then replaced by the only source of nutrition available.
Managing the medical needs of survivors with injuries and deteriorating health, using the limited medical knowledge of two students and no equipment beyond improvised bandages.
Organizing two larger expeditions and multiple smaller reconnaissance missions to understand the terrain around them and find a way out.
Building a sleeping bag capable of surviving overnight temperatures in the high Andes, without which the final trek would have been impossible.
Walking ten days through some of the most difficult terrain in the world to reach a herdsman with a note wrapped around a rock.
Waiting two and a half more days after that for the helicopters to come.
What is not on this list is also significant. What the survivors did not do, in 72 days, is give up. The documentation supports the conclusion that the group consistently, in the face of multiple setbacks and in the ongoing knowledge of their situation, continued to work toward survival rather than accepting that survival was impossible. The avalanche killed eight people and psychologically devastated the group; the survivors dug out and kept organizing. The radio failed; the survivors redirected their efforts toward the trek. Each individual death — there were 29 of them, and the survivors witnessed most — was a loss that had to be absorbed and then set aside in order to continue.