When does the play take place?

This play is set in "the present."  That is, it is set in the time when it was first performed.  Specifically, it is set in the year 1949.

The setting of the play is important for the themes that it is meant to explore.  For example, one of the major themes is that Willy Loman is up against forces that are larger than he is.  He is confronted with a society and economy that are changing.  During the late 40s and early 50s, the US economy was getting bigger and companies were more important.  Willy was not qualified to function in this economic reality.