It depends. For example, if each day you generate a random nonce and use it to salt that day's PII (and don't store the nonce) then you cannot later determine

It depends. For example, if each day you generate a random nonce and use it to salt that day's PII (and don't store the nonce) then you cannot later determine (a) did person A visit on day N or (b) is visitor X on day N the same as visitor Y on day N+1. But you can still determine how many distinct visitors you had on day N, and answer questions about within-day usage patterns.