I have a short question, when I entered Malaysia I was given a 90-day stamp but my return ticket was for 1 month, the officer didn't really say anything regarding that but he did write down the return date underneath the 90 day stamp, so my question is, would it be okay for me to change the return ticket closer to the 90 days and just explain to them later that I originally intended to stay for 1 month but then decided to stay longer? because I know they had a 1-month stamp but instead he stamped 90 days for me. I hope you can help me with this question.
The entry stamp in your passport indicates how many days (including the arrival date) that you are allowed to legally stay in the country.
You could stay 90 minutes or 90 days, it's 100% up to you. You do not need to inform anyone at immigration of anything. You just leave like you normally would.