No one can deny that having your own time machine would be pretty awesome. Who wouldn't jump at the opportunity to go back in time and hang out with some of the most important figures in history, like Abraham Lincoln or Tyrannosaurus Rex?
Unfortunately, even if you survive the accidental rewriting of history and/or destroying the space-time continuum, time travel will also make your own life pretty shitty as well.
To borrow from Professor Rufus of Bill and Ted University, "No matter what you do, no matter where you go, that clock, the clock in San Dimas is always running."
"However, time will never run out on these sunglasses."
So you get in your DeLorean and you leave 1985 to go on an adventure in the past. Say it takes you six months to accomplish your goal (ie, nearly making out with your mom) and when you're done, you go back to your own time. Maybe you go back to the very moment you left.
But you are six months older. There's no way around it. The time machine can't adjust your age backward--if it did, it would be altering your brain at the same time, wiping out the memories of what you experienced. No matter how many rejuvenation clinics from the future you visit, you will always age along your own timeline just as certainly as Marty did throughout Back to the Future.
Poor kid aged five years in one damn weekend.
When you get to the core of it, traveling through time is pretty much a deal with the Devil. Yes, it will enable you to save John Connor and get to Muggle Studies on time, but it will shave several months to several years off your life depending on how much you abuse it. These are precious moments from your twilight years that you will never get back: one last weekend with your wife, your granddaughter's graduation and the inevitable cloning of John Candy.
In short, when you've managed to hit age 89 in seven years and you're lying on your deathbed, will it really have been worth seeing everything time had to offer just to miss out on your own life? Sure, things may suck now, but you have no idea what it's going to be like in the future. By rewriting it all, you may just end up missing out on your own God-authored happy ending.
So let's say that somehow you've come into possession of Bill and Ted's time-traveling phone booth. Awesome, now you have the opportunity to find out for yourself what it was like when the Mongols ruled China, among other things.
Travel back to a time when Alex Winter and Keanu Reeves were the exact same level of famous.
After a run-in with Napoleon at Austerlitz, you decide to tackle the Western Movement in America in 19th century New Mexico. After some Reconstruction Era sightseeing, a saloon catches your eye. You are totally thirsty, and they are totally not carding!
Unfortunately by beer #2 and plate of wings #6, your insides are starting to feel like a sitcom on TBS. Why? It's kind of the same reason you don't want to drink the water if you take a vacation in India. There is, uh, stuff in it that the locals have gotten used to but you haven't.
Likewise, mankind's history of purifying food and water for the past thousand years or so has significantly weakened your modern stomach's ability to tolerate impurities, such as all the microbes, piss and yes, even shit you'll find in most foods and beverages from the past.
"Well you're just shittin' up a storm over there, aren't you?"
As a result, should you eat or drink anything prior to the publication of Upton Sinclair's The Jungle, your less-than-ironclad digestive system will start tearing itself apart like the flight crew in Event Horizon. So, unless you are willing to prep yourself in advance by consuming a well-regulated diet of historically-accurate shit sandwiches, we are sorry to report that you must abstain from all the delicious meals time travel has to offer you.
So you've got an HG Wells-esque time machine, which, if two separate movies are any indication, means you'll be able to fling yourself 800,000 years into the future. Think of all you'll have to learn by talking to the locals! Don't worry, they all speak perfect English, with only a slight accent. Language is pretty much the same, everywhere, forever, right?
Hmm.... let's try it the other way. Let's go backward in time, not 800,000 years, but just 1,000. You step out, grab some English reading material and find that it looks like this:
Well, this isn't right! After all, even the freaking monkeys in Planet of the Apes (SPOILER: It's future Earth) spoke plain English. What the hell?
Hollywood has mislead us, friends. Even though English is one of the most common languages on Earth, it gives you a splash-range of only a few centuries when it comes to linguistically "safe" places to travel. This is because the phonology of most languages is ever-changing, and it goes way beyond throwing in some "thee's" and "thou's" and referring to women as "my lady."
They aren't a "lady" if they wear pants.
No matter where you go, you'll have to speak both period and possibly even regional dialect to avoid coming off as a spy, a rival neighbor, a hillbilly or a crazy person who may or may not be possessed by the devil.
So unless you remembered to download a protocol droid app for your iPhone, your best bet is to pass as a mute no matter where you go, lest you open your mouth and risk getting into a situation that could potentially cost you your life. Even something as innocent as asking a peasant for directions could come off as unfathomably bizarre to your new peers. And in a great many places in a great many time periods, "unfathomably bizarre" gets your ass burned at the stake.
Not like you'd have anything to say to people anyway. After all...