They are, if it has been approved by the Sacred Congregation of Rites (S.R.C., 4597-4746).  But a priest travelling (peregrinus) should recite the office according to the calendar of the church to which he is attached regularly, but the obligation of following the calendar of his home church was not binding by a grave precept.  A reply of the Sacred Congregation of Rites (Nov., 1831) arranged (1) that beneficed clergy are always bound to recite the office of their own proper church or diocese; (2) that simple priests may read either the office as arranged for the place they tarry in or travel in, or the office of their own home diocese; (3) for unattached priests (vagi) it is the wiser order to follow the office as laid down in their own diocese.

Must every holder of a benefice read the Divine Office?

Every holder is bound, under pain of mortal sin, to recite the Divine Office daily, if the benefice be an ecclesiastical benefice fulfilling the conditions named above.  The omission of the recital of the Divine Office by a beneficed person is a grave sin against the virtue of religion and a grave sin against the virtue of justice.  For the Church imposes on the beneficiary the duty of the Office recital, on condition that he may not take the fruits of his benefice if he do not recite the Office.

What sin is committed by the omission of a notable part of the daily office?

He who wilfully omits a notable part of the daily Divine Office commits a mortal sin.  A notable part of the Divine Office for any day is held by some theologians to be the omission of one psalm in one of the small hours, or a corresponding quantity of matter in lessons, responses, etc.  They hold that such wilful omission is a grave sin.  Other theologians hold—­and their opinion is the more common and the more probable one—­that, although one psalm is a notable part of a small hour, in relation to the whole office it is not a notable part, and its omission is not a grave matter.  These theologians hold that the wilful omission of an entire small hour or equivalent matter (e.g., Sext, or the third nocturn of Matins) is an omission of a notable part and cannot be excused from grave sin.

The omission of the entire office of a day, the seven canonical hours, is held by some theologians to carry the guilt of seven mortal sins.  Because, there is a different precept for each hour and the omission of each hour violates a precept.  The Salamenticenses think this opinion probable.  The more common and the more correct opinion is that by such omission only one sin is committed.  And the theologians who hold this opinion say that the recitation of the canonical hours is imposed under one precept only, and hence there is only one obligation embracing the seven hours.  This is the opinion of St. Alphonsus (n. 148) who quotes several authors (including Lessius, Sanchez and St. Antoninus) in support.  If a person in Holy Orders omit several hours with a retractation, or a moral interruption in his sinful intentions, he may commit several mortal sins, because all the omissions, which in themselves are grave matter, may become independent of each other by the interruption and renewal of the intention (St. Alphonsus, n. 148).