Your description is not clear enough. It seems you have an unstored calculation on the child side of the relationship. This won't work.
If I understand correctly, the calculation checks if the date is within the last 12 months? If so, the solution is to move the calculation to the parent side of the relationship, and use only the (stored) date on the child side - for example:
Parent::cYearAgo < Child::Date
where Parent::cYearAgo is an unstored calculation of current date minus 12 months.