millis give negative results

In a sketch i need to count the time, so i've used millis. the problem is that when i calculate seconds (millis()/1000) when it reach 32 seconds it started to write -31, -30, -29 ....
What is my error?

millis() returns an unsigned 32-bit integer. You are treating it as if it were a signed 16-bit integer.
It would help if you post your code (in code tags - use the </> icon).