Article 12 of the General Data Protection Regulation (GDPR) creates a "one month" timescale for controllers to comply with data subject rights requests (such as subject access). But when does the time start? Until very recently the Information Commissioner's Office (ICO) said:
You should calculate the time limit from the day after you receive the request (whether the day after is a working day or not) until the corresponding calendar date in the next month. [wording taken from archived web page live on December 21 2018]
but now it says:
You should calculate the time limit from the day you receive the request (whether it is a working day or not) until the corresponding calendar date in the next month.
The change, which might seem minor, has been made with no announcement and is, in fact, potentially very significant (and costly) when one considers the number of systems which will have been set or programmed according to the earlier wording.
So what is the justification for the change? The answer lies in European Regulation 1182/71, determining the rules applicable to periods, dates and time limits and a 2004 court case involving the early marketing premium in relation to veal calves.
In Case C-171/03, Maatschap Toeters and M.C. Verberk v Productschap Vee en Vlees, the Court of Justice of the European Union considered the meaning of Article 3(1) and (2) of Regulation No 1182/71, and noted that the rule of law, common to many member states, of dies a quo non computatur in termino means that where a statutory provision expresses a period in days, weeks, months or years, the period is to be calculated from the moment at which an event occurs or an action takes place, and the day during which that event occurs or that action takes place is not to be considered as falling within the period in question. However, the time, so to speak, starts ticking from that point:
The dies a quo, or day during which the event took place, is therefore the day from which the period starts to run, and from which the period of time fixed by the law will be calculated.
As the court explains, this means, for instance, that "if an event which is the point from which a period of a week starts to run happens on a Monday, the period will end on the following Monday". This is reflected in the Rules of Procedure for Court of Justice, which also provide that the time finishes at the expiry of the last day of the period (i.e. 23:59).
This is how an apparently arcane law and a 15-year-old court case about cows led to a regulatory change which potentially affects almost everyone in the country.