Code not working like I want

Question

I have two date fields, "StartDate1" and "EndDate1".
When I make the EndDate1 value less than StartDate1, the "button" remains VISIBLE, but INACTIVE, and I can move on to the next field.
Why does the button remain visible, why is the button inactive, and how do I force the user to enter an EndDate1 value greater than that of StartDate1?
Also, in the Validate code, if I remove the line
" else xfa.host.messageBox("It Works"); "
then I get a syntax error, which I don't understand.
Can someone help clean up this mess?
Thanks,
Bernie
----- F.P1.EndDate1::exit - (FormCalc, client) --------------------
if ($.isNull ne 1) then
var nDays = Date2Num($.formattedValue, "MM/DD/YYYY");
if (nDays <= 0) then
xfa.host.messageBox("Please enter the date in MM/DD/YYYY format.");
xfa.host.setFocus($.name);
endif
endif
----- F.P1.EndDate1::validate - (FormCalc, client) ---------------
var DCV
if (HasValue(StartDate1) and HasValue(EndDate1)) then
DCV = Date2Num(EndDate1, "YYYY-MM-DD") - Date2Num(StartDate1, "YYYY-MM-DD");
// xfa.host.messageBox (DCV);
if (DCV<0) then
xfa.host.messageBox("End Date must be greater than Start Date");
xfa.host.setFocus($.name);
else xfa.host.messageBox("It Works");
endif
else $ = null
endif

Answer 1

Thanks for responding.
If I rename the back_btn instance to next_btn and the next_btn to back_btn and click the back button, this is the error:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at david_fla::MainTimeline/nextImage()
Any suggestions?