as.Date("03/04/2019")
#[1] "0003-04-19"

as.Date("03/04/2019", "")
#[1] "2019-07-16"

## S3 method for class 'character'
as.Date(x, format, tryFormats = c("%Y-%m-%d", "%Y/%m/%d"),
        optional = FALSE, ...)

as.Date("03/04/2019")
[1] "0003-04-20"

as.Date("03/04/2019", format = "")
[1] "2019-07-16"

as.Date("03/04/2019", format = "%d/%m/%Y")
[1] "2019-04-03"