With StatusBarSeconds you can add the seconds to the time that appears in the status bar of your iPhone, and you will see how it is updated second by second. But be careful, until its author warns, a function like this, with constant updating, can increase battery consumption, so we can not recommend it, the iPhone battery lasts long enough to install things that help to spend it.
You can download it free on Cydia.
You will find it in the BigBoss repo.
You need to have the jailbreak done.