The Cretaceous Is Always Doomed

Time Travel is here and it's time to go to the Cretaceous at last and observe some dinosaurs! But we gotta Race Against the Clock, because by the end of the day the asteroid is going to hit!

But wait, wasn't the Cretaceous period 79 million years long? Wouldn't it be better to go visit it any other day but the one guaranteed to kill you?

This is when The Theme Park Version of prehistory reveals its dark side for time travelers. No matter how sophisticated the method of time travel used, the arrivers will always have to complete their tasks before the asteroid arrives. There is no way around it, sometimes even if you're observing Jurassic or Triassic dinosaurs.

This need not apply only to the K-T extinction. Fiction loves to flanderize history into simple compact events and travelers headed to other periods may find themselves in the midst of other disasters, like arriving in Manassas only to find a civil war is breaking out, or visiting Roman Italy only for a volcano to erupt. Such a visitor is not looking to change history or see said famous event, they just want to take a stroll and breathe in the surroundings when... HOLY COW! This 20th century ship I'm on is called Titanic!

Related to Contrived Coincidence and In the Past Everyone Will Be Famous. May be part of a Phlebotinum Killed the Dinosaurs plot.

Examples of The Cretaceous Is Always Doomed include:

Examples involving the K-T extinction:

Film

Dinosaur: Since it's a dinosaur movie, we do get the obligatory meteor strike to kick off the plot, but the meteor damage appears to be confined to a relatively small area, and the dinosaurs manage to survive by migrating to the nearest fertile area. It's like they just assumed that giant meteors were a regular occurrence in the Cretaceous Era.

Literature

In Megamorphs 2, a hole in space-time causes the Animorphs to accidentally arrive one day before the asteroid hit. They get into a tussle with several warring Ancient Astronauts trying to colonize Earth, the losing species attempting revenge by diverting the path of a passing comet. The meteor's strike creates another hole that lets the kids return to the present. It was one of the weirder books.
Justified in Pathfinder. The protagonist Rigg can travel back in time by identifying the "path" of a living thing that had walked the land before. In a moment of urgency he picks the most recent path of an extinct animal he sees, which turns out to be fleeing the Colony Drop that rendered it extinct.
The Magic School Bus book In the Time of the Dinosaurs has them escape from the asteroid at the end of their day in the Cretaceous, though also after visiting other periods. It's averted in the corresponding animated episode however, where there is no asteroid because Ms. Frizzle sends them back 67 million years in the past.

Live-Action TV

In the first episode of Prehistoric Park, Nigel must collect a T-rex for the park before the asteroid hits. He handwaves this by saying that he want to get a specimen that would have died anyway so the timestream won't alter too much. However, he later revisits the period on other trips involving significantly less asteroids.
In an episode of Between the Lions, Walter and Clay Pigeon get transported inside a book about dinosaurs. At first they believe they've scared off a T-rex with their roars, but then Lionel turns the page and they realize the T-rex was actually running from the meteor.

Video Games

Chrono Trigger has you traveling to 65 million BC, and in the conclusion of that part of the plot, a giant asteroid falls to the planet. Justified in that the asteroid that wiped out the remainder of the Reptites is actually La vos.

Web Comics

In Dawn of Time, Dawn and Blue are returned back to their own time just as the asteroid is about to hit. They don't escape the asteroid, but they do manage to get away from the Grim Reaper.

Western Animation

Invoked in an episode of Justice League Unlimited: Lord Chronos punishes his goon Chucko by stranding him in the Cretaceous at the moment of the meteor's landing. At the site of the meteor's landing. He even gets off a Bond One-Liner.
Back to The Future also pulled this out, with Doc Brown and Verne stopping a meteor from hitting to save a dinosaur they befriended, only to find out it was that one, and without it, dinosaurs still rule the earth.

Other

While they don't often involve time travel, many dinosaur documentaries are fond of ending their stories by having the asteroid impact. (If said documentaries are made by The BBC it's a pretty safe bet that they'll use Stock Footage from Walking with Dinosaurs to save money.)
The ride DINOSAUR mildly averts this by claiming initially you would have gone on a tour of the peaceful early Cretaceous, but a rogue researcher changed your arrival date to just before the extinction event because he wants to rescue Aladar.

Examples involving other historical events

Literature

This happens many times during The Magic Treehouse series. It's justified because the books are usually meant to take them to a specific event like the Pompeii eruption, but Jack and Annie often don't bother to read further in the book until said disastrous event is happening.
In Doomsday Book, time traveler Kivrin arrives at the start of the Black Death epidemic of 1348, twenty years later than she intended to arrive. The explanation is that history resists people from going to any time but specific dates, hence why she must arrive in 1348.

Live Action TV

This is often the driving plot device of Doctor Who. The Doctor and his companions will arrive at some key moment in time just before a volcano explodes, a ship sinks or a war breaks out (and usually find an alien plot behind it).
The Time Tunnel. If the protagonists ended up in a place where a historic event took place, they always arrived just before said event occurred. Indeed, the very first episode sends them to the Titanic.
In The Twilight Zone episode "There's No Time Like the Past" this was being done intentionally at first, as a man goes back in time to attempt to warn the people of Hiroshima about a nuclear bomb in 1945 (hours before it hit), prevent the sinking of the Lusitania (hours before it was torpedoed), and kill Adolf Hitler before World War II. But when he decides to stop trying to change the past and go live in 1881, this trope still comes into play. He arrives the day before President James Garfield is assassinated, but decides to let it happen. Then it turns out he arrived a few days before a huge fire killed some children at the local schoolhouse, and he struggles with whether or not to prevent it, only to end up causing it when he does try to intervene.
Intentionally subverted in Quantum Leap, where Sam's time-traveling missions (as determined by an unknown entity) only involved fixing the lives of normal people, never celebrities. The closest that he came to do so was in one mission where he delayed Marilyn Monroe's death by a year, and another where he tried to prevent President Kennedy's assassination and succeeded only in saving his wife (who originally had also been killed).