How to change or inject date to Sharepoint application to test across months

Here is the situation: I've got the application that requires different actions for different months. For example if I'll create a task that requires some actions for next 5 months I need some kind of method to keep jumping ti next months to confirm correct behaviour. Usually I would just connect custom NTP for the server and than change date to whatever I want but this application works on Sharepoint online and as far as I know I can't do that (even if I'll it will affect all users not app under test only). Another problem is that method should be easy and possible to explain or at least show to business during UAT. Thanks in advance.