Calculating day since first date for ID
We are currently using FMP13 in clinical research. We have 3 tables at the moment (Patients Info, Patient Drugs and Daily Scores). In Patient Drugs, we record daily drug doses whilst Daily scores are daily recordings of scores. A portal is used to enter the relevant data. There can be multiple drugs/scores every day and each time we get a new portal row, a new timestamp is created. We wish to create a field that will calculate the study day which is the GetDate(timestamp) - the first date recorded for this patient. The first day will be study day 1.
I can just create this field and manually enter the study day but was wondering if there is an automatic way to do this as an calculation.