How to parse a mixed date format to be yyyy-mm-dd format in R?

[Dewi-Amaliah]

I have a dataset with a mixed date string format ("dd-mm-yyyy" and "yyyy/mm/dd"). I want to change it to be a uniform yyyy-mm-dd format. I have tried the lubridate package, but it resulted in NA in some rows (failed to parse). Anyone can help me to solve this problem? Here's I provided my code and the output.

library(tidyverse)
library(lubridate)
#> 
#> Attaching package: 'lubridate'
#> The following objects are masked from 'package:base':
#> 
#>     date, intersect, setdiff, union

date <- c("13-04-2015","2015/05/13", "13-06-2015", "2015/07/13", "13-08-2015", "13-09-2015")
freq <- c(24, 15, 36, 24, 17, 27)

dat <- tibble(date, freq)

dat %>% mutate(date = dmy(date))
#> Warning: Problem with `mutate()` input `date`.
#> x  2 failed to parse.
#> ℹ Input `date` is `dmy(date)`.
#> Warning: 2 failed to parse.
#> # A tibble: 6 x 2
#>   date        freq
#>   <date>     <dbl>
#> 1 2015-04-13    24
#> 2 NA            15
#> 3 2015-06-13    36
#> 4 NA            24
#> 5 2015-08-13    17
#> 6 2015-09-13    27

^{Created on 2020-08-13 by the reprex package (v0.3.0)}

Session info

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