Now is the time span between the past and the future. It can be long (like an eon in geologic time) or short (like a picosecond) but it is almost always used to refer to the span between the present instant to some time horizon when a decision must be made. It can be used to ask or demand that someone make a decision even if they want to delay.
"I want to know what you think, now."
"What do you think now?"
"Now is the time for all good people to come to the aid of their country."
Mathematics and measurement assume that everything used in one equation equals the same quantities at the beginning of calculation or axiomatization as at the end. That means it is mathematically correct to say that the idea of "equal" means "equal from the time the process starts to the time it ends." In General Semantics and E Prime the words equal, remain (for the past until now) and become (for now into the future) replace the verb "to be" for this reason.
Algebra is now often called snapshot algebra or algebra of seeing because of this dependence on time. If any action or event were possible between steps in algebraic analysis, then, in theory, one would have to start over as if one had no knowledge of the new state at all. For these reasons the idea of statistics and also knowledge and knowledge management are sometimes questioned, for instance, in the book Lies, Damn Lies, and Statistics. A major issue is the comparing of numbers gathered in the past, and now, after some key conditions change.