Ancient Egyptian Chronology
The history of Ancient Egypt has been pieced together by scholars from a number of sources, some more reliable than others. The Egyptians recorded the date of an event by noting how many years the current Pharaoh had been on the throne (regnal dating), and we don't know how long many of the Pharaohs reigned. Co-regencies and the overlapping of certain dynasties also added to the difficulty in verifying dates, so the orthodox chronology makes a number of assumptions in order to come up with the dates given below. However, these assumptions are not accepted by everyone.
The Orthodox Chronology
The Orthodox Chronology makes three important links between Egyptian history and that of her neighbours in order to establish a dating system.
The Sacking of Thebes
In 664 BC Thebes was sacked by the Assyrian king, Ashurbanipal in retaliation for a revolt led by Pharaoh Taharka (Dynasty Twenty Five). This date is confirmed in Assyrian, Babylonian, Egyptian text and is considered to be secure by most scholars.
The Bible (I Kings 14:25,26 and II Chronicles 12:2-9) records that a Pharaoh called Shishak attacked and defeated Jerusalem when Rehoboam was king of Judah. The Orthodox Chronology assumes that the Biblical Shishak is Shoshenk I of the (Dynasty Twenty Two), largely because their names sound alike. This is a major assumption, and has been questioned by many (for example David Rohl). It seems that archaeologists had misinterpreted the hieroglyphs recording one of Shoshenk's campaigns, and thought that he captured the kingdom of Judah. Shoshenk's campaign was in the Northern Kingdom of Israel, and there is no mention of Jerusalem. Once the assumption was made, scholars counted back from Rehoboam to the sacking of Thebes to calculate how many years had passed. They then used source material from Egypt to estimate the length of each Pharaoh's reign, and filled in the gaps.
The Exodus of the Israelites
Ramesses II (Ramesses the Great, Dynasty Nineteen) is cast as the Pharaoh of the oppression of the Israelites in Egypt. This assumption is based on the fact that the Bible states that the Israelites were forced to build a store in the city of Ramses, a city founded by Ramesses. This is not exactly proof. Genesis 47:11 also states that Jacob and the Patriarchs settled in the region of Ramesses, but this is ancient history to the Israelites of the Exodus. To support the argument, the dates gained from the assumption about Shishak are cited. However, in order to get the theory to fit the calculation, loads of the dates given in the Bible are amended. For example, two biblical sources (Judges 11:26, and I Kings 6) agree that the exodus took place around 1450 BC, but the Orthodox Chronology chops about 200 years off and states that Ramesses reigned from 1279 - 1213 BC!