When performing bulk in-life extension changes you can set a frequency of 1 monthly/quarterly rental but then set the dates to be up to a year long and the system will still accept.
In the system it will block you from selecting a date longer than the period entered, but if you use an upload template you can bypass this restriction.
As this can cause potential issues if someone was to make this mistake and not notice it, I think a validation rule, if possible would be appropriate so that the correct dates can be input and if anyone was to put a 1 month extension as several months / a year in the template, it would be rejected.
