Here is an extract from JAR-FCL 1 concerning the period during which it is necessary to pass 14 ATPL exams:
Subject to any other conditions in JARs, an applicant will be deemed to have successfully completed the required theoretical knowledge examination for the appropriate pilot licence or rating when awarded a pass in all of the required subjects within a period of 18 months counted from the end of the calendar month when the applicant first attempted an examination.
So taking this into account, it is possible to have 19 months to pass all the remaining ones (13, in case of a success during the first sitting) after first exam sitting in reality if you sit it on the first day of the month. For example, if you take first exam on the first of March, during the remaining part of this month the clock isn't ticking and will start doing so only on the first of April, so you get an additional month. Am I right?