Digital contact tracing apps allow to alert people who have been in contact with people who may be contagious. The Google Apple Exposure Notification (EN) system is based on Bluetooth proximity estimation. It has been adopted by many countries around the world. However, many possible attacks are known. The goal of some of them is to inject a false alert on someone else's phone. This way, an adversary can eliminate a competitor in a sport event or a business in general. Political parties can also prevent people from voting. In this talk, we review several methods to inject false alerts. One of them requires to corrupt the clock of the smartphone of the victim. For that, we build a time-traveling machine to be able to remotely set up the clock on a smartphone and experiment our attack. We show how easy this can be done. We successfully tested several smartphones with the German app (Corona-Warn-App), the Italian app (Immuni), the Swiss app (SwissCovid), etc.