Onchange In Asp.net Form Moves To Next Field?

Have an asp.net form where the textboxes have had the onblur attribute added. Though VS2008 still barks it's an invalid attribute...However, my problem is primarily w/the txtFICODate field.When testing using 10/10/2009 in this field, the code displays the appropriate msg based on format param. However, on exiting the isValidDate function in debug mode, I see that the code does not stop at the txtFICODate html but moves to the txtDOBB isValidDate function. Here the param is MM/DD/YYYY and it loops back into the function but now displays a different format message.Previous code (ValidateFICODate(obj)) below also caused the same looping error.

Debugging the same code with the txtDOBB field (10/10/56), I see it enter the function, display the appropriate message and exit. However, in debug, the code comes back to the function and then selects the field contents.how to fix problem so code stops at the txtFICODate field .Using xp Pro SP3, visual studio 2008 (c#).in the process of writing this, I searched other post here related to onblur + loop. So changed the event to onchange...and while the looping does not now occur, the focus just moves to the next field.

On page load code(if (!isPostBack)):
txtFICODate.Attributes.Add("onblur", "isValidDate(this, 'MM/DD/YY');");
txtDOBB.Attributes.Add("onblur", "isValidDate(this, 'MM/DD/YYYY');");

[code]....