The Rule of Istishaab (استصحاب)Istishaab means presuming that whatever you last knew for certain continues to hold, until you have clear evidence it changed.
The individual establishes the latest possible date they know for certain they were non-liable. Since certainty exists up to that point, Istishaab presumes they remained free of obligation until proven otherwise.
"Since your khums date recurs every year, the last certain date can't be more than a year ago, so walk backward from today, month by month. You know it wasn't last month. Five months ago, you're not sure. Eight months ago, you think it might have been, but you're still not certain. Nine months ago, you know for certain it was. Since nine months ago is the furthest point you're certain of, that becomes your last known khums date, and your next one falls exactly one year from it."
If you have never determined a khums date before, istishaab does not apply, since there is no prior known date to presume continues. In that case, you will need to set a new date going forward.
