This is a republication of a post on Compline which I had taken offline, butwhich I hope will be part of a series on the relationship between the early Roman and Benedictine offices.
The Nunc Dimittis and the Benedictine office
On the feast of the Purification, the Gospel reading contains the Nunc Dimittis, which is said daily at Compline in the traditional forms of the Roman Office. This canticle is not, however, said at that hour in the Benedictine Office.So why did St Benedict omit it?
One recent suggestion, from Jesse Billett, is that he didn't: rather the Nunc Dimittis was added to the Roman Office after the Rule of St Benedict was written. 
On the face of it the suggestion seems eminently plausible. There are, however, some reasons for questioning this conclusion.
The origins of Compline
It is worth starting by considering, by way of context, the origins of the hour of Compline.
Prayers before bed are mentioned in numerous early sources.
Whether these can be construed as references to proto-liturgical prayer though is contested.
Still, the key elements of Compline were clearly in place relatively early: its position as the 'hour' of prayer before sleep is set out in several early Office schemas; the use of certain fixed psalms at it seems to date from very early on; and the idea of an examination of conscience associated with it also has early origins.
The Apostolic Tradition (circa 225 or 375-400) , for example, lists the appropriate hours of prayer for the faithful as on rising at dawn/cockcrow (Lauds); before starting work (Prime); the third, sixth and ninth hours (Terce, Sext and None); before bed (Compline), again at midnight (Matins). 
St Ambrose (d397), in his instructions to Virgins, enjoins the use of psalms in conjunction with the Lord's prayer before sleep, as an aid to freeing the mind from earthly cares and focusing instead on the things of God. 
Similarly, St Basil (d379), in the Long Rule, includes prayers before bed in his listing of the hours and foreshadowed three of the key elements of what was to become monastic Compline, viz the use of Psalms 4 and 90 and an examination of conscience:
"The examination of our past actions is a great help toward not falling into like faults again; wherefore the Psalmist says: ‘the things you say in your hearts, be sorry for them upon your beds.’ (Ps 4:5)" Again, at nightfall, we must ask that our rest be sinless and untroubled by dreams. At this hour, also, the ninetieth Psalm should be recited. The psalms associated with the hour were apparently so well established known that the Ordo Monasterii associated with St Augustine (d430) just refers to 'the customary psalms before sleeping'. 
And while St Benedict's contemporary St Caesarius of Arles (d542) doesn't mention Compline in his Rule for Virgins (which he claimed followed the Office of the monastery of Lerins), his mid-sixth century successor as bishop of Arles, Aurelian (d551), did include the hour. 
For Italy, St Cassiodorus mentioned the hour as one of the seven hours each day alluded to in an office hymn attributed to St Ambrose.  It is St Benedict's Rule (c500-547), though, that contains the first detailed description of the hour. 
The Benedictine form of the hour was evidently used in Roman monasteries in the following centuries given the various references to the Rule in the Ordines Romani, including specific references to Compline in Ordo XVIII, which its original editor dated to the end of the eighth century, though others have convincingly argued dates much more likely from the mid-seventh century. 
The earliest surviving detailed description of the Roman version of the hour, however, dates from around three centuries later, and was penned by Amalarius of Metz (c775-850). 
The Roman hour and the 'organic development of the liturgy'
The hour Amalarius describes differs from the version St Benedict specifies in several respects: Amalarius doesn't repeat St Benedict's instruction that the psalms should be said without antiphon and 'directly' (ie without any repetitions of an antiphon or refrain); an additional psalm is included, namely the first six verses of Psalm 30; the hour is preceded by a reading (although not formally part of the hour itself); and it includes the Nunc Dimittis.
The table below compares the provisions of the Rule and the version of the hour described by Amalarius.
Modern Benedictine (1962)
Deus in adjutorium…
One of the difficulties with Billett's suggestion in relation to the Nunc Dimittis though, is that while the core of the Benedictine Office as laid out in the Rule did not (in most cases) change at all, or significantly, it does seem to have developed in parallel with the Roman Office, adding a number of additional features, such as collects. In the case of Compline, the modern version of the Benedictine hour includes both the opening reading, the confessional rite of the Roman hour, as well as the seasonal Marian antiphon after it (which has Benedictine origins). So why include those elements but not the canticle?
St Benedict's decision not to include Psalm 30 in his version of the hour, for example, probably goes in part to number symbolism: in his version of the Office Vespers and Compline together add up to seven psalms (a number symbolising completeness, as well as paralleling the seven days of creation), paralleling the number of psalms said at Lauds, and matching the symbolism of the twelve psalms said at Matins and again at Prime to None each day (the number of hours of the day).
It may also, though, reflect his preferred theological emphasises.
So was the omission of the canticle another such deliberate choice? Like Psalm 30, the Nunc Dimittis arguably serves to give Compline more of a flavour of the acceptance of death, rather than on repenting for our sins and resolution to do better in future, as urged by Psalm 4.
Te decet laus
There is another key reason for seeing it as a deliberate choice though, in a reference to a daily evening prayer consisting of an antiphon and the Nunc Dimittis in the fourth century Apostolic Constitutions:
You children, praise the Lord: praise the name of the Lord. We praise You, we sing hymns to You, we bless You for Your great glory, O Lord our King, the Father of Christ the immaculate Lamb, who takes away the sin of the world. Praise becomes You, hymns become You, glory becomes You, the God and Father, through the Son, in the most holy Spirit, for ever and ever. Amen. Now, O Lord, let Your servant depart in peace, according to Your word; for my eyes have seen Your salvation, which You have prepared before the face of all people, a light for the revelation to the Gentiles, and the glory of Your people Israel. Although the Apostolic Constitutions are almost certainly of Syrian origin rather than Roman, it does seem to imply the early use of the Nunc Dimittis as part of the customary prayers before bed, although they do not seem to have become part of any of the Eastern forms of Compline. Early on it appears to have been used at Matins; its use at Vespers seems to have been a thirteenth century development. 
All the same the Constitutions, (incorrectly) ascribed to Clement of Rome, were almost certainly known in Rome in the sixth century, since they were rejected as non-canonical by the Gelasium Decretal.  More significantly perhaps, the Constitutions provide our sole surviving source for the short doxological hymn, Te decet laus (see the bolding in the text above), that St Benedict specified be used in his version of Matins.
Jesse Billett, in The Divine Office in Anglo-Saxon England 597-c1000, made the suggestion that the Nunc Dimittis was a later addition to the Roman Office in order to explain its absence in an Anglo-Saxon Office manuscript including canticles and hymns thought to have originated with the Gregorian mission in 597. The traditional explanation was of course that the manuscript reflected the Benedictine origin of most of the missionaries: the book after all contains hymns, which were not used in the Roman Office, as far as we know, until the high middle ages. Billett, by contrast, sought to make the case that the missionaries bought the Roman Office (or at least Roman psalm cursus) with them rather than the Benedictine.
But given the continuing development of Benedictine Compline, presumably in response to the evolution of the Roman version of the hour, the traditional case seems to me at least as plausible as Billett's alternative suggestion. In particular, the circulation in Rome in Benedict's time of the Apostolic Constitutions, at the very least makes the Nunc Dimittis' association with evening prayer in the city by this time a strong possibility. And at the very least, St Benedict's probable familiarity with the Apostolic Constitutions implies that he made a deliberate choice not to use it in that context, instead repurposing the short doxology associated with it, just as he chose to drop Psalm 30 (assuming it too was already part of Compline) from his version of the hour.
 Jesse Billett, The Divine Office in Anglo-Saxon England 597-c1000, Henry Bradshaw Society, London, 2014, pp 114.
 Chapter 41 of the Apostolic Tradition (there is an ongoing vigorous debate on its dating and origins, on which see Ashbrook Harvey, Susan; Hunter, David G. (2008). The Oxford handbook of early Christian studies. Oxford University Press. p. 430).
 Ambrose On Virgins, Book III;
 Trans St. Basil, the Long Rules, tr. M. Wagner, New York, Fathers of the Church Inc., 1950 pp. 306-311.
 St Augustine's Ordo Monasterii. (Sr Michaele Puzicha, though, in her recent commentary on the Benedictine Rule argues that this is not a reference to collective prayer).
 Caesarius of Arles, Rule for Virgins, in Caesarius of Arles, Oeuvres Monastique, de Vogue and Courreau ed and trans, 2 vols, Sources Chretienne 345, 398; Aurelian of Arles, Rule for Monks, in Vincent Desprez, Adalbert de Vogüé (ed and trans), Règles monastiques d'Occident: IVe-VIe siècle, d'Augustin à Ferréol, Abbaye de Bellefontaine, 1980.
 Cassiodorus, Commentary on Psalm P118:164, in P G Walsh (trans), Cassiodorus: Explanation of the Psalms, Paulist Press, NY, 1991: “Should we wish to interpret this number [seven] literally, it denotes the seven offices with which monks in their devoted piety console themselves, namely, matins, terce, sext, none, vespers, compline, nocturn; the hymn of saint Ambrose, sung at the sixth hour, also attests this.”
 RB 17&18.
 Although Guy Hallinger argued that the Benedictine Rule was not used in Rome after Benedict, more recent assessments by have challenged this view: see in particular Marios Costambeys and Conrad Leyser, To be the neighbour of St Stephen: patronage, martyr cult, and Roman monasteries c, 600-900 in Kate Cooper and Julia Hillner ed, Religion, Dynasty, and Patronage in Early Christian Rome, 300-900, CUP 2007, pp 262-287, and Constant J. Mews (2011) Gregory the Great, the Rule of Benedict and Roman liturgy: the evolution of a legend, Journal of Medieval History, 37:2, 125-144.
For Ordo XVIII see Michel Andrieu (ed), Les ordines romani du haut moyen age, 1961, vol iii, pp 197-208. Andrieu argues that the Ordo as a whole deviates from the Rule in several respects but there is no obvious reason to view these as other than legitimate adaptations to the circumstances. Andrieu points out, for example that the Rule doesn't envisage saying the Office in the dormitory rather than the oratory, but the Roman basilican monasteries were often located at some distance from the church itself, and in some cases there seem to have been specific agreements (mentioned in the Liber Pontificis) as to which of the hours they would provide in the church itself. On its dating, see Mews above.
 Eric Knibbs (ed and trans). Amalar of Metz, On the Liturgy, vol II, Dumbarton Oaks, 2014, pp 376 - 383.
 Book VII , XLVIII.
 Gregory Woolfenden, Daily Liturgical Prayer Origins and Theology, pp 285.
 The authenticity and origin of collection of books deemed non-canonical in the Gelasian decretal continue to be debated: their original author suggested a late fifth century southern Gaul origin; Bronwen Neil has recently defended its authenticity.