# How to calculate the difference between two dates in JavaScript

By using the Date object to make calculations, we define a difference in years or days, and obtain a person's age

## Difference between two dates in years

Here is a JavaScript function that performs the calculation. It takes two dates and the second argument is the later. There is a simpler code that we will give, but this one provides access to more information about the dates.

For example, the age of the Eiffel Tower is **
** years, a number that is calculated using the following formula: *new Number((new Date().getTime() - new Date("31 March 1889").getTime()) / 31536000000).toFixed(0)*.

`function dateDiff(dateold, datenew)`

{

var ynew = datenew.getFullYear();

var mnew = datenew.getMonth();

var dnew = datenew.getDate();

var yold = dateold.getFullYear();

var mold = dateold.getMonth();

var dold = dateold.getDate();

var diff = ynew - yold;

if(mold > mnew) diff--;

else

{

if(mold == mnew)

{

if(dold > dnew) diff--;

}

}

return diff;

}

Demonstration:

Code of the demonstration...

```
var today = new Date();
var olday = new Date("1 January 2000");
document.write("Years since January 1, 2000: ");
document.write(dateDiff(olday, today));
document.write(" years.");
```

## Difference in days

For this case we convert years to days, the difference is simple.

```
function dayDiff(d1, d2)
{
d1 = d1.getTime() / 86400000;
d2 = d2.getTime() / 86400000;
return new Number(d2 - d1).toFixed(0);
}
```

Demonstration:

## Differences in years, simpler code

We use the previous code for the difference in days and divide by 365.

The code:

```
function dayDiff(d1, d2)
{
return new Number((d2.getTime() - d1.getTime()) / 31536000000).toFixed(0);
}
```

Demonstration:

## Calculating the age of a person

To view the age of a person or thing, the previous code is used again, with date of birth as the first parameter, but without a second parameter.

The code:

```
function age(birthday)
{
birthday = new Date(birthday);
return new Number((new Date().getTime() - birthday.getTime()) / 31536000000).toFixed(0);
}
```

Demonstration:

Code:

```
The age of Michael Schumacher is
<script>document.write(age("3 January 1969"))</script>
years.
```

Live demonstration