store previous date values in to a string using a <b>separator "&" and append</b> all new string values to the old string.
<b>split</b> the string with the character "&" and store it into an array of string.
<b>Get the count</b> of array and <b>create dynamic textbox controls</b> with respect to the count of the contents of the array.
include the above logic into your onclick code.