In Wide Sargasso Sea, Jean Rhys not only famously shifts the focus of the narative from Brontë’s Jane Eyre to Bertha Rochester (who here first appears as Antoinette, but she also sets the novel decades later than Brontë did hers. Drawing upon Mark McWatt's "The Preoccupation with the Past in West Indian Literature," Veronica Marie Gregg points to the novel's abundant evidence of this “crucial” change of chronological setting:
Marmion, newly published when Jane is given a copy toward the end of the novel, appeared in 1808, whereas the date for the Emancipation Act is 1833. Bertha Mason was already confined to the attic of Thornfield Hall by the first decade of the nineteenth century, whereas Antoinette Cosway in Wide Sargasso Sea was still a child in the 1840s. In terms of Jane Eyre the literary references in Wide Sargasso Sea are equally anachronistic: "Jeannie with the Light Brown Hair" was written by Stephen Foster in 1826; Tennyson's "Miller's Daughter" was not well known until the 1840s; most of Byron's poetry and all of Scott's novels (noticed by Antoinette's husband on the bookshelf at Granbois) appeared after 1800 ("Preoccupation," 12-13). These shifts and changes show the process by which Rhys deliberately locates the events of her novel in the West Indies of the 1830s and 1840s. 
In other words, whereas Jane Eyre takes place at a time when slavery provided the basis of West Indian economy and politics, Wide Sargasso Sea begins immediately after emancipation, and its narrator clearly wants us to sympathize with the effects of emancipating slaves had upon the Creoles or white former slaveholders.
The period Rhys in which the book has as much importance to its political themes and contexts as that in which she set it: She wrote Wide Sargasso Sea at precisely the period when Black Power and Afro-Caribbean writers and theorists seized and reconceived the history and cultural of the West Indies. Enthralled by the apparent postcolonial rewriting of Jane Eyre, few readers noticed that one of Rhys's chief concerns was most unfashioably providing a voice for the former slaveholders. Gregg, who elsewhere writes with fine clarity, makes this excellent point in prose so jargon-ridden that it threatens to plunge into self-parody:
Wide Sargasso Sea, as a rereading and rewriting of Jane Eyre, seeks to articulate the subjective and locational identity of the West Indian Creole of the postslavery period. It also imaginatively reinvents a category evacuated of social and political meaning in the 1950S and 1960s, the period of writing when colonial structures are being dismantled. Structurally and ideologically, then, Wide Sargasso Sea is deliberately anachronistic. . . . . Wide Sargasso Sea, a work in which the West Indies of the 1840s impinges upon and elucidates the England and West Indies of the 1950s and 1960s, seems to be underwritten by the Creole's desire to reclaim hegemony over the literary representation of the West Indies and "black people." The racialist usurpation of the voices, acts, and identities of "black people," so central to Rhyss writing as a whole, is the psychological cement in the architecture of this novel. 
- The Imagined Worlds of Wide Sargasso Sea and Jane Eyre: the Differing Roles of Religious Belief
Gregg, Veronica Marie. Jean Rhys's Historical Imagination: Reading and Writing the Creole. Chapel Hill and London: The University of North Carolina Press, 1995.
McWatt, Mark. "The Preoccupation with the Past in West Indian Literature." Caribbean Quarterly 28, nos. 1-2 (1985): 12-19.
Rhys, Jean. Wide Sargasso Sea. London: Andre Deutch, 1966.