alt Hacker NewsIn my experience with health data, the dates are usually offset by a random but constant amount for each person (e.g. id 12345 will have all their dates shifted by +5 weeks) to avoid identification by dates.
Unfortunately the sequence of treatments and locations are usually enough to identify someone, especially if it's a rarer condition.
Location data is very readily available, so you can easily correlate visits to a health facility with a treatment, and even with an offset, you can probably uniquely identify someone with 4 visits depending on the size of the medical facility.