The clock component from the sensor drawer does have several methods to work with date and time
A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook App Inventor 2 Book: Create Your Own Android Apps ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.
There is a free programming course here Course In A Box and the aia files for the projects in the book are here: App Inventor 2 Book: Create Your Own Android Apps
How to do a lot of basic things with App Inventor are described here: How do you...? .
The time picker is not enough.
You need the date picker too, and the Clock component's blocks to build an Instant from them, then convert to milliseconds from the Instant for your calculations.