This is the result provided by IntelliSense in Roblox.
Meanwhile, this is the API reference of it in devhub.
Difference:
Devhub is t2 first, then t1 meanwhile the IntelliSense is t1 first, then t2.
Intellisense displays it as number meanwhile the devhub displays it as int.
The correct answer should be int os.difftime ( int t1, int t2), I have no idea how can such a small mistake takes months.