How do I check the date form that without input?

There are two "Date Field".
If there is no input in one of them, I want to reset the other "Text Field", but I can't.

The following JavaScript is embedded in the "Text Field" that I want to reset.

In the example below, the "Text Field" is "AA_A".
The "Date Field" are "Day_Start" and "Day_Set".

By the way, "Text Field" is numeric.

And also readonly.

var AAAa = this.getField("AA_A");

var AAAb = AAAa.value;

var DayBa = this.getField("Day_B");
var DayBb = util.scand("yyyy/mm/dd", DayBa.value);
var DayBc = DayBb.valueOf();

var DayCa = this.getField("Day_C");
var DayCb = util.scand("yyyy/mm/dd", DayCa.value);
var DayCc = DayCb.valueOf();

All of the following do not work.

(1)

if (!DayBc || !DayCc ){this.resetForm("AA_A");};

(2)

if (DayBc == null || DayCc == null ){this.resetForm("AA_A");};

(3)

if (DayBc == undefined || DayCc == undefined ){this.resetForm("AA_A");}

(4)

if (DayBc == '' || DayCc == '' ){this.resetForm("AA_A");}

(5)

if (DayBc == 0 || DayCc == 0 ){this.resetForm("AA_A");}

Actually, I tried various other methods that I could come up with, but they were all useless.

For example,  AAAb = ''  on the right...

Thanking you for your advance.