Time Slip

To relate past experiences, a writer can choose the past tense or decide to tell the story in the present tense to add some immediacy and let the reader more directly participate in the action. Whatever the writer decides, however, he must be consistent.
It is possible to write a biographical piece in present tense and tell the reader that Jeff Bridges “comes into the movies in the 1970s.” However, the writer then cannot continue this sentence by referring to the 1970s as the time “when the battles over Vietnam were raging” ("The Dude Plumbs His Soul"/NYT 2/28). Both verbs (to come and to be) must be in the same tense, whether present tense to make the moment more immediate or past tense to indicate that the 1970s lie far behind us.
Danielle Pergament wrote down her own experiences while traveling through Tuscany. One of the local wine growers she met on the way was Mr. Brandli, who “speaks emphatically and at great, great length about the virtues of small farms. ‘Once you see firsthand how the food is made,’ he said, ‘it will taste different to you.’” He speaks cannot be followed by he said. Moreover, Mrs. Pergament clearly relates her experiences in the past tense, continuing, “As we were talking, half a dozen pigs, muddy and playful, came trotting up. These were the renowned cinta senese pigs, indigenous to Tuscany, named for the white belt around their bellies, and famously flavorful. Not that I could imagine eating one, once I saw how cute they were.” The last verb were, however, would need to be present tense since the cinta senese are cute in general, not just at the moment Mrs. Pergament saw them. General truths, however, are always expressed in present tense.
Therefore, that “Elliot Schrage, who oversees Facebook’s global communications and public policy strategy, said it was a good thing that young people are thinking about what they put online” is wrong. It is a good thing. Even in reported speech, general truths require the present tense, and it is a general truth that this heightened awareness is a sign of a growing responsibility for one’s “digital reputation” ("A Tell-All Generation Learns to Keep Things Offline"/NYT 5/9).
Ms. Henry, a new union leader, also shows a sense of responsibility. “I feel this ominous responsibility to take the union to the next level,” Ms Henry … said in an interview in her office, which was filled with union and feminist posters ("New Union Leader Sees Group as More of a Political Powerhouse"/NYT 5/9). In fact, her office is filled with posters since they most likely were not put up just to impress the reporter and taken down as soon as the interview was over.
But back to Tuscany and Mrs. Pergament’s travels. After tasting Mr. Brandli’s wine, she “drove to Bagno Vignoni, a medieval village built on thermal waters from an aquifer and popular since the Roman empire. The town square is a giant pool fed by volcanically heated water bubbling from the depths, steaming in the winter air, and the village has its share of day spas that use the water. A hot bath isn’t so appealing during an August heat wave, but on a blustery day in December, it was perfect.” The details about the town are correctly stated in present tense since they are general truths, and so is the fact that a bath in steaming water is not appealing in August. But by the same token it is indeed perfect in December ("Tuscany without the crowd"/NYT 3/7).
A perfect companion to Italian wine is classical music. For Robert Sadin, the producer of "The Art of Love," "medieval music and Machaut in particular represent a time when classical music had not yet become classical music. There is a certain mood of intense pathos that you see in most classical-music performances, and Machaut doesn't have that. His music seemed more in touch with the way we live; it seemed very close." If there is a connection between Machaut’s music and the way we live (now), then the music seems in touch with our life and seems very close - unless the music actually seemed to be in touch with the way people lived in the Middle Ages ("Forsaking 'Authenticity'"/WSJ 3/30).

A very detailed overview on the use of tenses can be found here.