Anna, I'm not sure which sources you checked, but I always try to find answers within HHS/Office for Civil Rights. Regarding dates being PHI, check out this link:
If you're using the Safe Harbor method to de-identify, the section I copied/pasted below states which dates need to be removed. However, I suggest reviewing the entire section...as Frank said, if you're relying on the Expert Determination method you should know the process so you can ask questions and understand your 'Expert's" decisions.
Safe Harbor method includes....
(2)(i) The following identifiers of the individual or of relatives, employers, or household members of the individual, are removed:
(C) All elements of dates (except year) for dates that are directly related to an individual, including birth date, admission date, discharge date, death date, and all ages over 89 and all elements of dates (including year) indicative of such age, except that such ages and elements may be aggregated into a single category of age 90 or older
It states, "all elements of dates (except year) for dates that are directly related to an individual." It goes on to state "including..." and lists some examples, but in my opinion that list is not all inclusive.