How to change POSIXct objects display in facets title? - r

Question

I am trying to create a ggplot2 graph using facet_grid(). Each facet has to be entitled with a date (here a POSIXct object) and I would like to change the way it is displayed.
How can i control the way POSIXct objects displays in ggplot2 facets title ?
Ex : this is how it is displayed : "2019-03-29"
and here is how I would like to see it written : "29/03/2018"
I have already looked at the labeller function but I can't figure out how to use it to change the way POSIXct object display. Maybe I am missing something.
I know facet labels can be "manually" changed but here I want a solution that works for any POSIXct object.
# create a dummy dataframe named ex
ex = structure(list(date = structure(c(1510531200, 1510531200, 1522195200,
1522195200), class = c("POSIXct", "POSIXt"), tzone = "UTC"),
cat = c("a", "b", "a", "b"), measure = c(0.0777420913800597,
0.71574708330445, 0.725231731543317, 0.217509124660864)), row.names = c(NA,
-4L), vars = "date", indices = list(0:1, 2:3), group_sizes = c(2L,
2L), biggest_group_size = 2L, labels = structure(list(date = structure(c(1510531200,
1522195200), class = c("POSIXct", "POSIXt"), tzone = "UTC")), row.names = c(NA,
-2L), class = "data.frame", vars = "date", indices = list(c(0L,
1L, 8L, 9L, 16L, 17L), c(2L, 3L, 4L, 5L, 10L, 11L, 12L, 13L,
18L, 19L, 20L, 21L), c(6L, 7L, 14L, 15L, 22L, 23L)), group_sizes = c(6L,
12L, 6L), biggest_group_size = 12L, labels = structure(list(date = structure(c(1510531200,
1522195200, 1543881600), class = c("POSIXct", "POSIXt"), tzone = "UTC")), row.names = c(NA,
-3L), class = "data.frame", vars = "date"), drop = TRUE), drop = TRUE, class = c("grouped_df",
"tbl_df", "tbl", "data.frame"))
# create a graph
plot_ex = ggplot(ex, aes(x = cat, y = measure)) +
geom_bar(stat = "identity") +
facet_grid(.~date)
print(plot_ex)
The facets are named "2017-11-13" and "2018-03-28". I want them to be "13/11/2017" and "28/03/2018".
Many thanks for your help,

Answer 1

You can change how dates are printed with format. Using that, we can set an appropriate labeller, without changing the data.frame column.
ggplot(ex, aes(x = cat, y = measure)) +
geom_bar(stat = "identity") +
facet_grid(.~date, labeller = function(x) format(x, '%d/%m/%Y'))

Answer 2

We can use strftime.
ex$date <- strftime(ex$date, format="%d/%m/%Y")
library(ggplot2)
plot_ex <- ggplot(ex, aes(x=cat, y=measure)) +
geom_bar(stat="identity") +
facet_grid(.~date)
print(plot_ex)