Have a look at these, all:

Here are three uses of now from Dictionary.com that I feel are pertinent to this discussion:

At the present time: goods now on sale; the now aging dictator.

At this point in the series of events; then: The ship was now listing to port.

At times; sometimes: now hot, now cold.
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Possible sentences for each (sorry, you'll have to provide your own wider contexts and cotexts):

At the present time: goods now on sale; the now aging dictator.

"Now" in past tense environments:

"I met the dictator by the pool filled with coy carp, his passion. Now aged since last we met, he greeted me as a prodigal returned and a long cold shiver ran down my back knowing the things he's been up to recently."

"Now" in present tense environments:

"Here we are today July 21st, 2004, at the silver jubilee celebrations of General ---- , supreme dictator and beneficent father of the island of ... . The General, now well aged, is standing tall in his finery of..."
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At this point in the series of events; then: The ship was now listing to port.

"The Romans had stormed into Britain in 43AD. Now, 453 years later, they drizzled out like a fine summer rain."

At times; sometimes: now hot, now cold.

"Steve did a double-take. He saw the strange lights shifting erratically-now purple, now green; now bright, then... dim. Gone!"
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Happy to answer any questions.