Mathematically calculating a longer VIP period poses certain challenges and will bring a lot more complexity into the process.
In the below case, what should a donation of $20 do? 1 month Gold + 1 month Bronze? Should he have an option to take 4 month bronze? or two month silver?
Additionally, most streamers have a different way of resetting their VIP status and very few offer options to scale the VIP length based on donation amount.
These were the reasons to choose a default period that can be customized rather than offer a mathematical day count.