Difference between dates is 1 day short

Question

Can someone please tell me why the result of this script is one day short? (The date fields are formatted mm/dd/yyyy)

Ex: the difference between 01/01/2019 and 12/31/2018 == 0 instead of == 1

//dateEnd

var actual = this.getField("date.Actual.Closing").value;

var dateEnd = util.scand("mm/dd/yyyy", actual); //get date object for end date

//Start date: construct it from dateEnd as 12/31/ of the yyyy before dateEnd

var yr = this.getField("date.YearPrior");

var year = dateEnd.getFullYear();

yr.value = year - 1

var strStart = this.getField("date.LastDec31");

strStart.value = util.printd("mm/dd/yyyy", new Date("12/31/" + yr.value));

var dateStart = util.scand("mm/dd/yyyy", strStart.value);

//Calculate and assign the date difference

var dDiff = dateEnd.getTime() - dateStart.getTime(); /

var oneDay = 24 * 60 * 60 * 1000;

var days = this.getField("num.days");

days.value = Math.floor(dDiff / oneDay);

Test Screen Name · Accepted Answer

I'd be wary of using floor, because you are assuming that the millisecond values are held and calculated exactly, and I'd worry about leap seconds.

I suggest you display the actual values of dateEnd.getTime() and dateStart.getTime(), to see if that's the problem.

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