In that case, it looks like this field represents the number of days from a particular starting point, a common date encoding from Excel. If so, you can convert the numbers to dates by specifying the appropriate origin date.
dates <- c(NA, 31418, 34799, 34799, 34799)
as.Date(dates, origin = as.Date("1899-12-30"))
#> [1] NA "1986-01-06" "1995-04-10" "1995-04-10" "1995-04-10"
Created on 2020-06-01 by the reprex package (v0.3.0)
You can also take a look at the janitor package, which has a function that does this too: