Today is only one day in all the days that will ever be. But what will happen in all the other days that ever come can depend on what you do today. It’s been that way all this year. It’s been that way so many times. All of this war is that way.
The passage emphasises how a single day can matter far beyond its immediate duration. Robert Jordan recognises that present actions may influence events that unfold much later, particularly in war, where individual decisions can affect operations, lives and wider outcomes. The importance of the moment comes not from the day itself, but from the consequences attached to what is done within it.
In For Whom the Bell Tolls, this idea reflects Jordan’s awareness that the mission before him may influence more than his own fate. The quotation also has a broader meaning outside warfare. Many important outcomes emerge from a sequence of decisions whose significance may only become clear afterwards. Hemingway therefore presents the present as both ordinary and consequential: today is one day among many, yet it can become a turning point if later events depend upon the choices made within it.