Last year for Christmas my mom bought me a copy of The Purpose of Christmas by Rick Warren (he also wrote the popular book, The Purpose Driven Life). The other night while I was reading it, he mentioned something that I had never really thought of before:
"You may have never realized that every time you check your calendar or refer to a date or write one down, you are using Jesus Christ as your reference point. Because of Jesus, history is divided into BC (before Christ) and AD (anno Domini, in the year of the Lord). Every other event in history and every event on your calander today is dated by how many days and years it has been since Jesus Christ appeared on earth. Even your birthday is dated by His birthday."