You don't need to use the static keyword.

The cleanest solution is to define DateTime outside of the loop function:

DateTime now;

This will allocate the memory, and call the constructor of the class DateTime initializing it.

In case you would need a basic variable (like an int), you should initialize it, like:

int now = 0;

When you use it in setup or loop you use the created instance (without the class name to declare it):

void loop() {
   ...
   now = rtc.now();
}

Also, find a better name for now, because the variable is the same as the rtc's function now.

You don't need to use the static keyword.

The cleanest solution is to define DateTime outside of the loop function:

DateTime now;

This will allocate the memory, and call the constructor of the class DateTime initializing it.

In case you would need a basic variable (like an int), you should initialize it, like:

int now = 0;

When you use it in setup or loop you use the created instance (without the class name to declare it):

void loop() {
   ...
   now = rtc.now();
}

Also, find a better name for now, because the variable is the same as the rtc's function now.

Update

after your comment, it cannot be used as a global variable:

I didn't try it myself, but I suspect, the constructor calls a function that uses variables or a function that is not initialized at that point. Actually, it's quite bad practice.

Anyway, one way to circumvent and stick as much to what I wrote above, is to use a pointer instead:

RTC_PCF8523* rtc = NULL;

Than you create an empty pointer, without calling the constructor. You will do this at the end of setup:

void setup() {
   ...
   rtc = new RTC_PCF8523();
}

This will cause the variable to be put on the stack.

Than when using it, you call:

now = rtc->now();

Note you have to use the -> notation, as rtc is a pointer.