Six changeful years have vanished since I first
Poured out (saluted by that quickening breeze
Which met me issuing from the City's walls)
A glad preamble to this Verse: I sang
Aloud, with fervour irresistible
Of short-lived transport, like a torrent bursting,
From a black thunder-cloud, down Scafell's side
To rush and disappear. But soon broke forth
(So willed the Muse) a less impetuous stream,
That flowed awhile with unabating strength,
Then stopped for years; not audible again
Before last primrose-time.

I have italicised the clauses which give some clue to the dates of composition. From these it would appear that the "glad preamble," written on leaving Goslar in 1799 (which, I think, included only the first two paragraphs of [book first]), was a "short-lived transport"; but that "soon" afterwards "a less impetuous stream" broke forth, which, after the settlement at Grasmere, "flowed awhile with unabating strength," and then "stopped for years." Now the above passage, recording these things, was written in 1805, and in the late autumn of that year; (as is evident from the reference which immediately follows to the "choir of redbreasts" and the approach of winter). We must therefore assign the flowing of the "less impetuous stream," to 1802; in order to leave room for the intervening "years," in which it ceased to flow, till it was audible again in the spring of 1804, "last primrose-time."
A second reference to date occurs in the [sixth book], p. 224, entitled "Cambridge and the Alps," in which he says,

Four years and thirty, told, this very week,
Have I been now a sojourner on earth.

This fixes definitely enough the date of the composition of that part of the work, viz. April 1804, which corresponds exactly to the "last primrose-time" of the previous extract from the [seventh book], in which he tells us that after its long silence, his Muse was heard again. So far Wordsworth's own allusions to the date of The Prelude.
But there are others supplied by his own, and his sister's letters, and also by the Grasmere Journal. In the Dove Cottage household it was known, and talked of, as "the Poem to Coleridge;" and Dorothy records, on 11th January 1803, that her brother was working at it. On 13th February 1804, she writes to Mrs. Clarkson that her brother was engaged on a poem on his own life, and was "going on with great rapidity." On the 6th of March 1804, Wordsworth wrote from Grasmere to De Quincey,

"I am now writing a poem on my own earlier life: I have just finished that part of it in which I speak of my residence at the University." ... It is "better than half complete, viz. four books, amounting to about 2500 lines."[A]

On the 24th of March, Dorothy wrote to Mrs. Clarkson, that since Coleridge left them (which was in January 1804), her brother had added 1500 lines to the poem on his own life. On the 29th of April 1804, Wordsworth wrote to Richard Sharpe,

"I have been very busy these last ten weeks: having written between two and three thousand lines—accurately near three thousand—in that time; namely, four books, and a third of another. I am at present at the [Seventh Book]."

On the 25th December 1804, he wrote to Sir George Beaumont,

"I have written upwards of 2000 verses during the last ten weeks."

We thus find that Books [I.] to [IV.] had been written by the 6th of March 1804, that from the 19th February to the 29th of April nearly 3000 lines were written, that March and April were specially productive months, for by the 29th April he had reached [Book VII.] while from 16th October to 25th December he wrote over 2000 lines.
Dorothy and Mary Wordsworth transcribed the earlier books more than once, and a copy of some of them was given to Coleridge to take with him to Malta.
It is certain that the remaining books of The Prelude were all written in the spring and early summer of 1805; the [seventh], [eighth], [ninth], [tenth], [eleventh], and part of the [twelfth] being finished about the middle of April; the last 300 lines of [book twelfth] in the last week of April; and the two remaining books—the [thirteenth] and [fourteenth—before] the 20th of May. The following extracts from letters of Wordsworth to Sir George Beaumont make this clear, and also cast light on matters much more important than the mere dates of composition.