What must a person do who has a doubt that he has omitted something in his recitation of the office? Is he bound to make assurance doubly sure by reciting the part of which he doubts?
If the doubt be a positive doubt, that is, if he have good reason to believe that he has recited it, he is not bound to anything further regarding the part in question. For instance, if a priest remembers having started the recitation of a lesson, and in a short time finds himself at the end of it, and cannot be sure if he have recited it, the presumption is in favour of the priest and of the recitation, because it is his custom to recite completely whatever part he commences. He has, thus, moral certainty that he has satisfied the precept, and it is not necessary to repeat it; if the necessity for repetition be admitted in such a case, a fruitful source of scruples is opened up.
On the other hand, if the doubt be negative—that is to say, if a person has no reasonable motive for believing that he has recited the full office or the full hour, he is bound to recite the part omitted, because in such a doubt, the precept of recitation is, as the theologians say, “in possession.” (St. Alphonsus, n. 150).
It is not allowed to change anything nor to add anything to the daily office without permission. The Sacred Congregation of Rites (10 June, 1690, n. 3222) replied to a query, that in saints’ offices nothing is to be added and nothing is to be changed, and this reply applies to all sorts of offices, old and new.
In reciting the Divine Office two points of order are to be noted: (1) the order or arrangement of offices, (2) the order or arrangement of Hours. The order of offices indicates which office is to be said on each day as laid down in the calendar. The order of the Hours points out which of the seven hours should be recited, firstly, secondly, etc., Matins, Lauds, Prime, Terce, etc. It is of obligation to observe both orders. But is it a sin to change wilfully the order of the office? It is not, if there be a reasonable cause for the change. For instance, if a priest cannot say the office proper to his diocese on a certain day, but says some other approved office, the change is not a sin. But if a priest, ex industria, substitute one office for another, it is per se a venial sin; but if an office be said which is very much shorter than the calendar office, or if this changing or substituting be so frequent as to disturb gravely the good order of the year’s offices, the sin may be (and, according to some authors, is) a mortal sin.