The problem is that "What is one month after 2021-02-28?" doesn't have a single meaning in plain human language.
It could mean 30 days in the future. Or the same day of the week 4 weeks in the future (i.e., 28 days in the future). Or the day of the same cardinality in next month. Or any date in the next month. And those are all equally correct.
It's simply not a precise measure of time when spoken from one human to another human in plain language. Indeed, I think we inherently understand it to be an imprecise measure of time just as much as "tomorrow" doesn't mean "exactly 86,400 seconds from this moment". "Next week" doesn't necessarily mean "7 days from now", either. That's why computers don't typically use imprecise terms. They provide feedback and say "this will occur at this time and date".
You always have to check what the operators actually do and what the requirements actually mean when you're working with times and dates.
It could mean 30 days in the future. Or the same day of the week 4 weeks in the future (i.e., 28 days in the future). Or the day of the same cardinality in next month. Or any date in the next month. And those are all equally correct.
It's simply not a precise measure of time when spoken from one human to another human in plain language. Indeed, I think we inherently understand it to be an imprecise measure of time just as much as "tomorrow" doesn't mean "exactly 86,400 seconds from this moment". "Next week" doesn't necessarily mean "7 days from now", either. That's why computers don't typically use imprecise terms. They provide feedback and say "this will occur at this time and date".
You always have to check what the operators actually do and what the requirements actually mean when you're working with times and dates.