When topping up online, the Maxis system will only give active time, according to the amount paid, from the date the top-up is made. In contrast, the active time I pay for should be added to whatever existing active time is still available rather than deleting it and starting the new time from the date the top-up is made.
For example, if I have a week's worth of time available, and I top-up a week early, then that time is lost. The system will delete it. Currently the only way to get the time you pay for is only top-up on the last day of active time, then you will get the time you pay for. If you pay for time in advance, then you will lost the time already paid for.

I believe the programmers could easily allow the system to extend the time from the last active day rather than just overwriting the existing time. I believe it is only to the financial advantage for Maxis to have this unfair billing process in place.

Please be aware of this Maxis practice when they take your money on a prepaid plan.