make date field visible then invisible depending on value selected in dropdown list

I used the following code in Properties Calculate to make a date field visible if "yes" is the selected choice from a dropdown list. The code works perfectly until the form is opened in Acrobat from a smart device, in my case an iPhone. The date field then fails to become visible when "yes" is selected in the dropdown list. All help with this appreciated as I have 30 date fields in my form

if (this.getField("approved1").value == "yes"){  

event.target.display = display.visible;

}else

{

event.target.display = display.hidden;

}