Here are the dates in a roughly chronological order with a discussion of the importance of the date and what kinds of problems I usually encounter.
6:00 pm, Saturday, October 23, 4004 BC
This is the date calculated by Archbishop James Ussher in the 17th Century. If you would like to see how he calculated this date, you can read an extensive analysis the Conservapedia.com: Date of creation. I am more than aware of the various controversies that surround this date and the religious fervor the various positions engender, but genealogists really shouldn't be worried about extending their pedigrees "back to Adam." There is no benefit that accrues from spending the time copying one or more of the unsupported genealogies into your own records whether you believe the date and the religious justification for the date or not. If you want to see my summary of the issue then you can view my YouTube.com video entitled, "Why You Can't Trace Your Lineage Back to Adam."
Following up on the "Back to Adam" issue is the significant date of 1500 AD. This date is a rough approximation of the earliest time during which records of ordinary people would have been kept in Europe. There are a very, very few records that go back further, but most of the records that have genealogical significance are not available before this date. Whenever, I make this statement, I always have some that argue that "they have found more information about their family that goes back even further than this date." With a few notable exceptions, these people are merely copying from books published long after this date giving various pedigrees for kings and other prominent historical figures. Almost uniformly, these "older pedigrees" are based on undocumented and unsupported extensions.
There are some "proven" pedigrees that purport to extend the royal ancestry of immigrants to America, for example, but so what? When you begin copying, you are no longer doing genealogical research. If it makes you feel important or whatever to connect to European royalty, then go for it. But don't think you are impressing me or anyone who knows anything about these pedigrees.
I admire those who have the historical background, language skills and dedication to do Medieval research, but I have only met a small handful of these people in my entire life and they are certainly not the folks putting their family trees online. Note, there are a number of places in the world, such as China, where family records and pedigrees go back much further than 1500 AD. I might also mention that there are a few older parish records from Spain, but for most of Europe, this is about the limit.
This is the date that is commonly accepted as the beginning of the keeping of English parish records. If you have any questions at all about this date and want to show me how you got a baptismal record for you ancestor before this date, I would be glad to review it. Here is a book to get you started with your investigations in this area. If you don't know this history, you have no business adding content to online family trees. Keep your poor research and speculations to yourself.
Cox, Charles. The Parish Registers of England, by J. Charles Cox,... London: Methuen, 1910.
You can read a very nice, digitized copy of this book on the Archive.org website at https://archive.org/details/parishregisterso00coxjuoft. There are extensive references concerning the earliest records that have been located for each of the English parishes. Meanwhile, as soon as FamilySearch.org fixes the Family Tree and disconnects it from new.FamilySearch.org, I will be editing out all of the unsupported dates in all of my family lines.
1492 or 1620 AD
This is the date of the arrival of the Mayflower in America. Although some European settlements, especially those initiated by Spain, go back to the original time of Columbus. Arguments about earlier European settlements in America have an academic interest but not for genealogists.
1620 to 1847 AD
This is the range of dates when the first European settlers entered the area now segmented into the states of the United States of America. Every state and every county in the United States has a date when the first European (or whatever) settlers first entered that portion of the country. Usually referred to as the date of earliest settlement, these dates are commonly ignored by people who locate their ancestors in parts of the country that were not settled at that time. I ran across a reference to one of my ancestors who was supposed to have been born in Utah in the 1700s. This is a really common error. There are lists of the time of formation of every state and every county, try the Newberry Atlas of Historical County Boundaries for a start.
Obviously this issue blends into the date calculations that seem to show children born before their parents and after their parents died. But I see all of this as a symptom of the same problem; a total lack of awareness of history and dates. Before any of us can take seriously much of what passes as genealogical research, we will have to clean up the mess caused by this gross ignorance of history.
I could go on with dates such as the dates for the U.S. Civil war, and many others but I think this list gives an idea of the process we should all be aware of when we start putting our research online.