Dates of a time series from Excel to R

General
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2

Use as.Date() and specify the origin

dates <- 42917:42918
as.Date(dates, origin = "1899-12-30")
#> [1] "2017-07-01" "2017-07-02"

If you need more specific help, please provide a proper REPRoducible EXample (reprex) illustrating your issue.