we use ajax calendar in asp .net , when we insert date of birth in data base if textfield is blank then it take default value, how we remove this default value?

First of all you need to add some more detail to your question, like what value you need to store as a default value etc..

FOR some help see this...

For this you need to care for it at the time of inserting the data..

When inserting data and at that time if birth date text box is empty or black then you need to pass the value what you want to store when the text box is empty..

Like when text box is empty you need to store NULL then while inserting you need to pass NULL value to your insert query