I haven't read the novel yet but according to the online sources, the events in The Martian take place in 2035 in the novel. I've watched the movie for about 7-8 times till now but couldn't find any clue in the movie about the date. I did carefully inspect many of the computer screens in it where none of them did include any date in them except the mentions of "Sol" counts and month and day information but no mention of the year again. I admit that there can be some screens which I may have missed. For example, the mission display of the Iris probe includes Mars and Earth time counts in it but the resolution at those frames doesn't allow to read what they are actually displaying. The only computer screenshots that I can't be sure of are the Chinese ones.
Also, the team members of the original Pathfinder missions don't look like that they are in their 70s or 80s. If the movie was taking place in 2035 they should certainly be about around 70-80 at best.
Every detail in the movie makes it look like that the events are taking place around our current timeframe.
So, in short, why did they hide the year in the movie or make it look like it's at furthest taking place in around 2020 or something.