Do clerics commit a sin if they do not recite the liturgy of the hours?
The short answer is yes.
First of all, we must distinguish among those who have an obligation to pray the Divine Office.
All Latin-rite priests and transitional deacons have the grave obligation, undertaken in the moment of ordination, to pray the entire Liturgy of the Hours: "Are you resolved to maintain and deepen a spirit of prayer appropriate to your way of life and, in keeping with what is required of you, to celebrate faithfully the Liturgy of the Hours for the Church and for the whole world?" (cf. Roman Pontifical, Rite of the Ordination of Deacons)." This is confirmed by No. 29 of the General Instruction of the Liturgy of the Hours and Canon 276, §2.3, of the Code of Canon Law.
With respect to the extent and possible exceptions to this general rule, the Congregation for Divine Worship offered the following clarifications:
"Question #1: What is the mind of the Congregation for Divine Worship and the Discipline of the Sacraments regarding the extension of the obligation of celebration or reciting daily the Liturgy of the Hours?
"Response: Those who have been ordained are morally bound, in virtue of the same ordination they have received, to the celebration or the entire and daily recitation of the Divine Office such as is canonically established in canon 276, § 2, n. 3 of the CIC, cited previously. This recitation does not have for its part the nature of a private devotion or of a pious exercise realized by the personal will alone of the cleric but rather is an act proper to the sacred ministry and pastoral office.
"Question #2: Is the obligation sub gravi extended to the entire recitation of the Divine Office?
"Response: The following must be kept in mind:
"A serious reason, be it of health, or of pastoral service in ministry, or of an act of charity, or of fatigue, not a simple inconvenience, may excuse the partial recitation and even the entire Divine Office, according to the general principal that establishes that a mere ecclesiastical law does not bind when a serious inconvenience is present;
"The total or partial omission of the Office due to laziness alone or due to the performance of activities of unnecessary diversion, is not licit, and even more so, constitutes an underestimation, according to the gravity of the matter, of the ministerial office and of the positive law of the Church;
"To omit the Hours of Morning Prayer (Lauds) and Evening Prayer (Vespers) requires a greater reason still, given that these Hours are the 'double hinge of the daily Office' (SC 89);
"If a priest must celebrate Mass several times on the same day or hear confessions for several hours or preach several times on the same day, and this causes him fatigue, he may consider, with tranquility of conscience, that he has a legitimate excuse for omitting a proportionate part of the Office;
"The proper Ordinary of the priest or deacon can, for a just or serious reason, according to the case, dispense him totally or partially from the recitation of the Divine Office, or commute it to another act of piety (as, for example, the Holy Rosary, the Stations of the Cross, a biblical or spiritual reading, a time of mental prayer reasonably prolonged, etc.). - What Should Be Prayed in the Liturgy of the Hours
Canon Law states the following:
Can. 276 §1. In leading their lives, clerics are bound in a special way to pursue holiness since, having been consecrated to God by a new title in the reception of orders, they are dispensers of the mysteries of God in the service of His people.
§2. In order to be able to pursue this perfection:
1º they are first of all to fulfill faithfully and tirelessly the duties of the pastoral ministry;
2º they are to nourish their spiritual life from the two-fold table of sacred scripture and the Eucharist; therefore, priests are earnestly invited to offer the eucharistic sacrifice daily and deacons to participate in its offering daily;
3º priests and deacons aspiring to the presbyterate are obliged to carry out the liturgy of the hours daily according to the proper and approved liturgical books; permanent deacons, however, are to carry out the same to the extent defined by the conference of bishops;
4º they are equally bound to make time for spiritual retreats according to the prescripts of particular law;
5º they are urged to engage in mental prayer regularly, to approach the sacrament of penance frequently, to honor the Virgin Mother of God with particular veneration, and to use other common and particular means of sanctification.
Father Edward McNamara, professor of liturgy at the Regina Apostolorum university explains it this way:
During the development process for the 1983 Code of Canon Law it was decided to remove expressions such as “under pain of mortal sin” with respect to the external prescriptions of Church law.
In part this was done to distinguish Church law and the moral law. Church law covers the external relationship of individuals in the Christian community. Since sin also involves internal factors, the law, in itself, does not bind under pain of sin.
This technical distinction does not mean that no sin is committed by transgressing Church law. The fact that the code no longer binds attending Sunday Mass under pain of mortal sin does not change the fact that willful and inexcusable absence is mortally sinful. - Obligation of the Liturgy of the Hours