function checkDisabled() {
	// disable back button
	if (year == 1985 && month == 11) {
		$("#nav_prev").addClass("disabled");
	} 
	// disable forward button
	if (year == 1995 && month == 12) {
		$("#nav_next").addClass("disabled");
	} 

	// undisable forward button
	if (year != 1995 || month != 12 && $("#nav_next").hasClass("disabled")) {
		$("#nav_next").removeClass("disabled");
	} 
	// undisable back button
	if (year != 1985 || month != 11 && $("#nav_prev").hasClass("disabled")) {
		$("#nav_prev").removeClass("disabled");
	}
}
// check whether to disable navigation buttons
check whether to disable navigation buttons
check whether to disable navigation buttons
function checkDisabled() {
	if (year == 1985 && month == 11) {
		$("#nav_prev").addClass("disabled");
	} 
	if (year == 1995 && month == 12) {
		$("#nav_next").addClass("disabled");
	} 
	if (year != 1995 || month != 12 && $("#nav_next").hasClass("disabled")) {
		$("#nav_next").removeClass("disabled");
	} 
	if (year != 1985 || month != 11 && $("#nav_prev").hasClass("disabled")) {
		$("#nav_prev").removeClass("disabled");
	}
}
function checkDisabled ( ) { if ( year == 1985 && month == 11 ) { $ ( "#nav_prev" ) . addClass ( "disabled" ) ; } if ( year == 1995 && month == 12 ) { $ ( "#nav_next" ) . addClass ( "disabled" ) ; } if ( year != 1995 || month != 12 && $ ( "#nav_next" ) . hasClass ( "disabled" ) ) { $ ( "#nav_next" ) . removeClass ( "disabled" ) ; } if ( year != 1985 || month != 11 && $ ( "#nav_prev" ) . hasClass ( "disabled" ) ) { $ ( "#nav_prev" ) . removeClass ( "disabled" ) ; } }
check whether to disable navigation buttons
check whether to disable navigation buttons
// disable back button // disable forward button // undisable forward button // undisable back button