Why is time separated into the time before and after Jesus was born and died?

It is commonly known point assumed by many. Why try to created something new and then teach everyone about it!

It isn't.  It's separated before and after his birth.  Who else in the world has made a greater difference for good?

What alternative is available that would satisfy everyone?

Makes you think, doesn't it. Various man-made calendars have been used throughout the centuries. Even today, the Jewish religious calendar is adhered to, and the Islamic calendar also. Yet they still work with the Gregorian (Christian) calendar for practical purposes in every-day secular life. In Islam they have Arabic names for months and this is the Hijira calendar, or AH for the Latin 'Ano Hegirae' (in the year of the Hijira).  1437 AH = 14 October to 2 October 2016 in the Gregorian calendar. Other religions still maintain their religious dating systems but it is true that the whole world basically follows the Gregorian (Christiann) calendar that divides history into "Before Christ" (B.C.) and "After Christ (A.D. = the Latin 'Ano Domini 'In the year of our Lord'). A lot of people who don't have Christ as their Lord prefer to use C.E. for 'Common Era'.

But yes, it's amazing, and it makes you think.

Because Jesus is the son of God and our Savior.  Seriously.  Would a story about a carpenter in a tiny corner of the world have grown and continue to have such an impact nearly 2000 years later if it were untrue?  The Bible is no fairy tale.  See some important facts in the link below. 

why is day separated from night?

same reasoning.

salvation brings light.

darkness does not.