I need to calculate an elapsed time between two date objects

Question

I have a date object created from vars saved in a database.

var prevTime = Date(year,month,day,hour,minute);

I want to calculate the difference between this and the current time.

var thisTime = Date();

I am doing this:

prevTime.getTime() - thisTime.getTime();

It gives me a negative number that is very large. I divide by 1000 to get seconds and then divide by 3600 to get hours. I need an elapsed time in hours. I end up with a number that is like -756.00. If the current time is larger than the previous time, why is the number negative? What am I doing wrong?

Thanks,

Scott

Answer 1

The current time is larger than the previous time so subtracting a larger number from a smaller number gives you a negative number. Reverse the order if you want the positive difference in times. Is there more to the question than that?

Demonstration here: http://jsfiddle.net/jfriend00/NYSsp/

var prevTime = new Date(2011,1,1,0,0);  // Feb 1, 2011
var thisTime = new Date();              // now
var diff = thisTime.getTime() - prevTime.getTime();   // now - Feb 1
alert(diff / (1000*60*60*24));     // positive number of days

Answer 2

First one must be smaller that you get negative. Your first date is in the past.

Here is an example:

var a = new Date(2011,7,5,2,1,1) - new Date(2011,7,5,1,1,1);
alert(a); // 3600000 which is millisecond for 1 hour. Divide by this to get hours.