Office challenge: Why is Excel displaying a date instead of a numeric value?

This week, test your Excel skills by solving this common but peculiar and annoying formatting snafu.

You don't always get what you expect when using Excel. Most of the time, users just shrug and work around the unexpected, if they know how. Occasionally, they call you because they just don't have the experience to avoid the problem.

Suppose a user calls you after entering the numeric value 90 because Excel displayed the date 3/30/1900 instead of 90. Even the formula bar is displaying a date-Excel ate her value! She swears that she didn't format the cell to display a date, and you believe her.

You can help her correctly display the numeric value quickly enough; even she knows how to do that. But can you explain why Excel displayed a date instead of a value? By helping her expose why this happened, you'll help her learn how to avoid this common, but unexpected formatting surprise.