Calculate future dates based on 1st date within related reco

I am working on a database for clinical trials having tables for Patients, Trial info, Patient in trial and then Services. (other tables not important here.)

Patients in trial table uses the Patient ID and the Trial/study ID as forgeign keys. This sets up each patient within a specific trial since patients may be in one or more trials. Services is a child of Patients in trial and lists the many followup dates and services done within the study. If all patients are seen at the same timepoints (day 1, day 15, day 30....for example) how can I calculate this so that the future dates will automatically enter as new records once I enter the first service date for each patient?