If you only want elapsed days, date fields are simpler to use than timestamps which record both date AND time and store that as a value in seconds where dates store the value as a value in days.
If you set up a field in the Patients Info field as a date field, it can auto-enter the creation date as your earliest date from which you want to compute elapsed days. You can also define a calculation field as Min ( RelatedTable::Date ) and select date as the result type to get a date in PatientInfo that is the earliest date in a related table such as Patient Drugs or Daily Scores.
Either way, a calculation field defined in the child table can subtract a date in that table from the date in Patient Info to compute elapsed days:
DailyScores::Date - PatientInfo::StartDate
IF you use timestamps instead of dates, the difference will show the number of elapsed seconds and you'll need to either specify a TimeStamp result type or divide the difference by the number of seconds in a day (24 * 3600).