Harold Wentworth, American Dialect Dictionary (1944) notes the phenomenon of using "whenever" to mean simply "when" (that is, "at the time that") in two regions of the United States—West Virginia and the Ozark region of southwestern Missouri and northwestern Arkansas—with instances cited from 1917 through 1943:
whenever, adv., conj. When, at the time that;—used in declarative sentences. Cf. everwhen [identified elsewhere in the dictionary as meaning "whenever" in the Ozarks area].
Among the examples Wentworth cites are this one from the Ozarks (in 1926):
'Whenever I was a gal, folks kep' ther' clo'es on, an' the men-folks allus wore the britches.'
and this one from West Virginia (in 1942):
'Whenever the teacher took up [Shakespeate's plays...]' = at the time that..
and this one from northwestern West Virginia (in 1943):
'My growth was stunted whenever I was small.' Boy age 10. Repeated.
If you're wondering how people express the idea of "whenever" in regions where the word whenever commonly means "when," you may find Wentworth's entry for "ever and when" interesting:
ever and when. Whenever, everywhen. 1933 s.w.Mo.—n.w.Ark. I'll go t' work ever an' when I git good an' ready.
It is tempting to suppose that "ever and when" (or "everywhen") might form a natural pair with "whenever" in any region where people commonly use "whenever" to mean "when," but that doesn't match my experience. As a child in southeast Texas, I occasionally encountered kids who used "whenever" in place of "when"—but I don't remember ever hearing anyone use "ever and when" or "everywhen" to mean "whenever."