Confusing error message for date fields

[Douglas Cerna]

If the user enters an invalid date, like 2011-02-29, the custom data converter for date fields raises an exception with a kind of confusing error message:

The datetime string did not match the pattern yyyy-mm-dd

First, I'm not comfortable with the 'datetime' part. These are dates... and I think an invalid date like this matches the pattern anyway. It's just invalid.