Setting Dates to Null Values based on Values list of Radio Button

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I am having Values List of Radio Button(4 values) Field in 1st interview screen. I am having 3 dates fields which are Enabled & Disabled based on Values List of Radio Button. I am selecting 1st value of radio button and based on that dates are enabled and user will enter dates and will go 3rd screen. Now, he will come to 1st screen and change Value from 1 to 2 from that radio button(before submit). For 2nd Value of that radio button,2 date fields will be disabled and 1 will be enabled.  Date Fields are disabled and now problem is