At http://www.messiahalive.net/ there is a link called "A Biblical Timeline of History" that goes through Scripture and counts how many years seem to have passed since creation.  I don't just try and say "This is it", but try to delve into several possible interpretation and say "It's at least this long" and "possible as long as this much" rather than tryign to be narrow minded and only examining it from one perspective.

 

We are told in Galations 3:17 tells us there was 430 years from God's promise to Abraham until the Torah was given at Mt. Sinai.  Josephus says it was 430 years EXACTLY, being 215 years TO THE DAY from Abrahams departure until the Jews became slaves, and 215 years TO THE DAY from the time they became slaves until they left Egypt.

Some have attempted to say there were 400 years from some post-Joseph period until the Exodus based on Gen 15:13, which says Isael would be slaves for 400, but this seems to disagrees with Galatians 3:17, Josephus, and other Scriptures. Also, Exodus 12:40 reads differently in different translations. The Masoret Torah says, "the length of time the Israelites lived in Egypt was 430 years". But the Dead Sea Scrolls, the Samaritan Torah, and the Septuagint read, "the length of time the Israelites lived in Egypt and Canaan was 430 years." So the Masoret text does not include the time spent in Canaan but most other texts do. Galatians 3:17 seems to agree with the Dead Sea Scrolls, Samaritan Torah, and Septuagint over the Masoret reading.

Also, if we trace geneology from Abraham to Moses, which is also given, it only fits about a 430 year time frame from Gen 12 to Exodus 20. The geneology is given all the way down to Moses and just doesn't see to allow for enough time for Israel to have been enslaved for 400 years under the Egyptians during this time period (click here to get more detail on that issue).

The "400 years" God spoke of in Gen 15:13 was probably not a 400 year contiguous period, but the sum of all periods of slavery Israel underwent to Egypt, Babylon, the Phonecians, etc.

In no way does this account for the fact that the Jewish calendar seems to be about 200-250 years off the correct answer.  I don't think confusion over how long Israel was in Egypt explains it.  I think there is another source for the gap.

Shalom, Joe