r - How to add layers with new data to a ggplot with a POSIX axis? -

i'm used if want add point ggplot, works fine:

ggplot(mtcars, aes(x = disp, y = mpg)) + geom_point() +     geom_point(x = 200, y = 20, size = 5, color = "blue") 

but, problems if there's posix dates involved:

dat_1 <- data.frame(time = as.posixct(c("2010-01-01", "2010-02-01", "2010-03-01")),                      y_1 = c(-1, 0, 1)) 

the basic plot works, of course

(my_plot <- ggplot(dat_1, aes(x = time, y = y_1)) +     geom_point()) 

but adding layer

my_plot + geom_point(x = as.posixct("2010-01-01"),     y = 0, size = 5, color = "blue") 

returns error

error in ops.posixt((x - from[1]), diff(from)) :    '/' not defined "posixt" objects 

converting numeric solves issue:

my_plot + geom_point(x = as.numeric(as.posixct("2010-01-01")),     y = 0, size = 5, color = "blue") 

but not necessary if mapping in aes wrapper

point_data <- data.frame(x = as.posixct("2010-01-01"), y = 0) my_plot + geom_point(aes(x = x, y = y), data = point_data,                      size = 5, color = "blue"