Hi, Please try this and let me know
function validate_calendar() {
var textbox = document.getgetElementById("your_textbox_id");
var cal_extender = document.getgetElementById("your_calendarExtendor_id")
if(textbox.disabled)
{
cal_extender.disabled = true;
}
if(!textbox.disabled)
{
cal_extender.disabled = false;
}
}