A common attempt to protect privacy is to truncate dates to just the year. For example, the Safe Harbor provision of the HIPAA Privacy Rule says to remove “all elements of dates (except year) for dates that are directly related to an individual …,” this restriction exists because dates of service can be used to identify people, as explained here.
Unfortunately, truncating dates to just the year ruins the utility of some data. For example, suppose you have a database of millions of individuals and you’d like to know how effective an ad campaign was. If all you have are the dates to the resolution of years, you can hardly answer such questions. You could tell if sales of an item were up from one year to the next, but you couldn’t see, for example, what happened to sales in the weeks following the campaign.