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Inconsistent behaviour with Javascript Date getTime() function

I am trying to make a filter based on hours for timestamps (in this example filter for all times after 8 am):

    var beginningTimeValue = new Date('2020-01-01 08:00:00');
    var unique = [["value","value","value","value","12/01/2021 00:03:35","value"],["value","value","value","value","01/01/2020 00:03:35","value"], ["value","value","value","value","01/01/2020 08:03:35","value"], ["value","value","value","value","01/01/2020 13:03:35","value"]]
      
    if(!beginningTimeValue == ""){
      unique = unique.filter(function(row)
                             {
        var rYear = row[4].substring(6, 10);
        var rMonth = row[4].substring(3, 5);
        var rDay = row[4].substring(0, 2);
        var rHour = row[4].substring(11, 13);
        var rMinute = row[4].substring(14, 16);
        var rSecond = row[4].substring(17, 19);
        var bTime = new Date(parseInt(rYear, 10), parseInt(rMonth, 10), parseInt(rDay, 10), parseInt(rHour, 10), parseInt(rMinute, 10), parseInt(rSecond, 10));
        console.log("ODATE = " + rYear + "/" + rMonth + "/" + rDay + "_" + rHour + ":" + rMinute + ":" + rSecond);
        console.log("BDATE = " + bTime.getFullYear() + "/" + bTime.getMonth() + "/" + bTime.getDate() + "_" + bTime.getHours() + ":" + bTime.getMinutes() + ":" + bTime.getSeconds());
        beginningTimeValue.setYear(bTime.getYear());
        beginningTimeValue.setMonth(bTime.getMonth());
        beginningTimeValue.setDate(bTime.getDate());
        if(bTime.getTime() >= beginningTimeValue.getTime()){
            console.log(bTime.getTime()*24*3600*1000 + " VS " + beginningTimeValue.getTime()*24*3600*1000);
        }
        else{
          console.log("FALSE");
        }
        return bTime.getTime() >= beginningTimeValue.getTime();
      }
                            );
    }
    console.log(unique);

I have debugged my way to finding out that I wouldn’t get a FALSE value in the 2nd IF, however I am at a loss as to why the .getTime() function returns vastly different values for my console log:

“136556220816000000000 VS -5.0438323821312e+21”

“136560264336000000000 VS -5.0438323821312e+21”

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Answer

The problem is in the following line:

beginningTimeValue.setYear(bTime.getYear());

The (deprecated) getYear() function returns

[a] number representing the year of the given date, according to local time, minus 1900.

The (also deprecated) setYear() function

[…] interprets any two-digit number as an offset to 1900

In your case, getYear() returns a value like 121, which is not a two-digit number. When you subsequently invoke setYear() with that value, you get a date that is set to the year 121 instead of 2021.

Since getTime() returns the number of milliseconds since 1970, and 121 is before 1970, you get a negative number.

TL;DR: use getFullYear() and setFullYear() instead of getYear() and setYear().

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