Every variable has it's own set of cases where it can be less reliable, so using any single variable will be prone to problems. For instance, "person.metrics.ownership" would be the variable that most consistently corresponds to days spent in your household, except that it actually counts days in your slave list. This means that time away for quests, vacations, training, or recovery still counts towards days of ownership.
Newly generated slaves don't really have any backstory and stores and slave auctions don't particularly constrain themselves to the rules of the world. Though only a few versions ago this was basically true for any time a slave was away from the mansion. Even now you can indefinitely lengthen the duration of a pregnancy by simply using the Dream spell on the slave every night.