How to count the seconds in Java?

I'm trying to understand how I could go about keeping track of the seconds that an object has been created for.

The program I'm working on with simulates a grocery store.

Some of the foods posses the trait to spoil after a set amount of time and this is all done in a subclass of an item class called groceryItem. The seconds do not need to be printed but are kept track of using a currentTime field and I don't quite understand how to count the seconds exactly.

I was looking at using the Java.util.Timer or the Java.util.Date library maybe but I don't fully understand how to use them for my issue.

I don't really have a very good understanding of Java but any help would be appreciated.

You can use either long values with milliseconds since epoch, or java.util.Date objects (which internally uses long values with milliseconds since epoch, but are easier to display/debug).

// Using millis
class MyObj {
    private final long createdMillis = System.currentTimeMillis();

    public int getAgeInSeconds() {
        long nowMillis = System.currentTimeMillis();
        return (int)((nowMillis - this.createdMillis) / 1000);
    }
}

// Using Date
class MyObj {
    private final Date createdDate = new java.util.Date();

    public int getAgeInSeconds() {
        java.util.Date now = new java.util.Date();
        return (int)((now.getTime() - this.createdDate.getTime()) / 1000);
    }
}