Inconsistent behaviour with Javascript Date getTime() function

June 21, 2022

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() + " VS " + beginningTimeValue.getTime());
    }
    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:

"1613084615000 VS -58345116808000"

"1580558615000 VS -58377689608000"

>Solution :

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().