Change Ubuntu time and date for specific application
You can use faketime to tell processes it's a different time.
Install it via:
sudo apt install faketime
Then just run your application like this:
faketime -f '-5d' date
This calls date. So just substitute date with whatever you like.
The example goes 5 days back change that part to whatever you like.
If you want to learn more about how you can use faketime, check out its man page:
man faketime
Note that this only works after you installed it.
If you're curious how to achieve this for an application you launch from the Dash, check out this answer.