James Ussher, born in 1581, is famous for calculating the day that God began creating the universe: October 23rd, 4004 BC (that's September 21st on our Gregorian calendar). While it certainly is not heretical to suggest that creation began on October 25th, it is important to realize that Scripture is not silent when it comes to the age of the universe. The Scriptures contain numerous genealogies and other time frames which can be pieced together in order to come up with an estimate creation date.
Scripture does not leave open genealogical gaps. In the earliest and most unknowable periods of history, from Adam to Noah (and Noah to Abraham to Joseph), Scripture traces a continuous genealogy and records when each person was born and when each person died.
After Joesph, Scripture records the number of years the Israelites were in bondage to the Egyptians, which leads into their 40 year wandering and ultimately leads into the book of Judges (which records the different lengths of Israel's captivity and peace). After the time period of the book of Judges, Israel begged God for an earthly king. God granted them their wish and from this point the length of each king's ruling is recorded (By this time period, other writings can be found which record additional historical events, of which, just as the Bible, the date can be known).
Clearly, Scripture is not silent concerning the age of the earth. While believing the earth to be around 6000 years old is not a popular belief today, it must be noted that Scripture is absolutely correct in all that it states. The God who made all of creation and who designed the laws of physics and all the materials in the universe, is surely intelligent enough to know when and how He created the universe.