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Is that because you're confusing some variables. for example: using the one containing the match bonus instead of the one containing the time.

Read the Tutorial re buttons:
How to Use Buttons, Check Boxes, and Radio Buttons (The Java™ Tutorials > Creating a GUI With JFC/Swing > Using Swing Components)

How are you doing this?
How do you detect the button press?
When/where do you record the starting time?
Where do you compute the duration from the starting time?
Where do you display that...

Does it work now? If not, please explain what it does or doesn't do.

Try debugging your code by adding printlns statements to show the execution flow and where variables are changed.

Why did you pick a complicated one instead of a simple one? Where did you get the code about the property change? Too much IDE generated stuff that beginners don't know how to read and use.
You...

Work on one small problem, solve it and then move on to the next one.
Programming can be very tedious and requires a lot of attention to detail. Lots of details.
Go slowly one step at a time until...

Have you read the API doc for the JTextArea class?
What methods does it have that will change the contents of the JTextArea?

There's a problem with your code. When I run it I get:
java.lang.NullPointerException
at ExpApplet.<init>(ExpApplet.java:111)

First get your code to execute w/o errors.

Then fix the GUI so...