Whilst I agree that the 18 month period to gather all the TK exams together is effectively until the end of the month as laid out in FCL.025(b)(2), I am not at all sure that this can be rolled forward onto the date for the issuance of a CPL (or IR or EIR).
FCL.025(c)(1)(iii) clearly states "...shall be counted from the day when the pilot successfully completes the theoretical knowledge examination..."
My interpretation is that the 18 month TK completion period starts at the end of the first month that the candidate attempted his/her first examination. The period for the issuance of the CPL etc is, however, a separate matter and relies on the date of completion of the TK exams.
Or to put it another way, the 18 month period explicitly starts at the end of the month of the first attempt. The 36 month period explicitly starts from the day of TK exam completion.
Happy to be proved wrong. Perhaps if someone has successfully obtained a CPL, IR or EIR more than 36 months after the date of completion of TK exams but up to the end of that month, they could let us know.