[...] date of composition is probably 1170/80-12OO; ... So Probably not St. Bernard who died in 1153.
As the first and most reliable MSS are English and as the use of the poem spread from England, it is reasonable to conclude that it was written in England. The anonymous English writer was probably a Cistercian. Whoever he was, he was well versed in the Scriptures and their liturgical uses and applications, and acquainted with the writings of St Bernard and with his use of the Scriptures, especially of the Psalms and the sapiential books. These reasons suggest a Cistercian.

The original poem was of forty-two verses, the text of which can be found in the Oxford Medieval and, with slight changes, in Wilmart. However, copyists and adapters changed the order of verses, omitted, altered and added verses, as they wished. Altogether the MSS show eighteen new verses and nineteen doxologies
or quasi-doxologies. These make a composite text of seventy-nine verses-apart from twenty-five variations of original or added verses which in the process have become almost new verses. This confusion in the MSS was not detected for some time so that Mabillon's edition, for instance, and the compilers of this Office in 1721 treated
as original some of the additions. If Roman numerals are used for the verses of the original and Arabic for the additions, the Breviary hymns are made up as follows:
41: verses I, II, III, V and 74;
42: verses IX, 14, IV, XII and 79;
43: verses XVIII, XVI, XXIII, X and XXXI.
Only in 41 are the verses chosen in anything like a consecutive order, and 42 is a very mixed affair. Hymn for the Transfiguration is also derived from this poem. But there the centoist strung together not verses but unconnected lines and in so arbitrary a fashion that the relation to its source would scarcely be suspected. The revisers made the relationship even more remote.