We could only know this if God revealed it to us. Short of noticing a moon-sized asteroid hurtling straight at us, we otherwise have no way to know.
Of course, there have been many people throughout history who claim that God
has revealed it to us (usually within the books of Daniel and Revelation), but in some obscure fashion that we would eventually figure out and make known to others.
Recently, as you probably recall,
Harold Camping made such a claim (twice), and was proven false.
Edgar Whisenant wrote a book in 1988 titled,
88 Reasons Why the Rapture Will Be in 1988. When 1988 passed, he went on to make a half dozen more failed predictions (next book:
Living on Borrowed Time).
In the 1840’s, William Miller made several failed predictions which were contorted into what is now known as the Seventh Day Adventist Church.
Even Martin Luther chimed in in the early 1500’s, but without quite as much precision:
One thing that seems common to all “end of the world” predictions is that the end is always imminent. I’ve never heard of anyone who “read the signs” and “did the math” and concluded that the world would end in the year 6250. It seems these guys all start with the assumption that the end is near and then try to fit the data to support their preconceived idea.
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People have been diligently “reading the signs” and “doing the math” for 2000 years, and nobody has gotten it right. To me, that’s a pretty strong indication that God has not actually revealed this to us, in any way.