See the last argument within hc_tooltip(). I've added xDateformat = " " which removes the date string at the top of the pointer.
# packages
library(highcharter)
# data
dataset <- data.frame(
Date = as.Date(c("01-01-2021","03-04-2021","01-12-2021"),
format = "%d-%m-%Y"),
Variable = c(1000, 2000, 1500)
)
# plot
plot <- hchart(dataset,
"column",
hcaes(x = Date, y = Variable)) |>
hc_xAxis(dateTimeLabelFormats = list(month = "%d-%m-%Y"),
type = "datetime") |>
hc_tooltip(pointFormat = "{point.x:%d-%m-%Y} <br> Variable: {point.y}",
xDateFormat = " ")
plot