If you want to increment a local variable, like "t" in your example, you must use the assignment operator ":" (read aloud "becomes" or "gets") like so:
t:t+1
Local variables exist only within the lifespan of a single event.
Persistent state, like an inventory or a system for tracking a user's action, must reside in a widget somewhere. This has been discussed many times and places within this forum. I recommend starting with this thread: https://itch.io/t/2702720/how-to-have-interaction-between-cards