Adding date ticks to ggplot in R - r

Question

I am trying to add ticks to my x-axis in this graph to show all the months of the year:
My code is as follows:
library(ggplot2)
library(scales)
p <- ggplot(df_test, aes(time, reading))
p + geom_point(alpha = 1/4) + geom_smooth()
I have tried to use scale_x_date but have come across the following error:
Error: Invalid input: date_trans works with objects of class Date only
Here's the data frame I'm using:
hour reading date time
1 53 1/1/15 2015-01-01 01:00:00
2 55 1/1/15 2015-01-01 02:00:00
3 56 1/1/15 2015-01-01 03:00:00
The class of my time variable:
class(df_test$time)
"POSIXct" "POSIXt"

Answer 1

Use scale_x_date(breaks="month", labels=date_format("%b%)). Here's an example.
library(quantmod)
sp500 <- getSymbols("SP500", src="FRED", auto.assign=FALSE)
sp500 <- sp500["2015-01-01::"]
sp500 <- data.frame(time=as.POSIXct(index(sp500), origin="1970-01-01"),sp500)
class(sp500$time)
# [1] "POSIXct" "POSIXt"
library(ggplot2)
library(scales) # for date_format(...)
ggplot(sp500, aes(x=as.Date(time), y=SP500))+
geom_line()+
scale_x_date(breaks="month", labels=date_format("%b"))

Answer 2

You are trying to use a scale specific for Date on a POSIXct object. The solution is to cast the POSIXct object to a date using as.date:
> Sys.time()
[1] "2015-09-16 09:52:42 CEST"
> as.Date(Sys.time())
[1] "2015-09-16"
To do this on your data.frame, I recommend using the dplyr package:
df_test = df_test %>% mutate(time = as.Date(time))

Answer 3

Combining the answers of both #PaulHiemstra and #jihoward, I've managed to figure out an answer.
First using the dplyr library to rework the data:
library(dplyr)
df_test1 = df_test %>% mutate(time = as.Date(time))
Then using scale_x_dates:
library(ggplot2)
library(scales)
p <- ggplot(df_test1, aes(time, reading))
p + geom_point(alpha = 1/4)+
scale_x_date(breaks="month", labels=date_format("%b"))
Gives the result:

Answer 4

Since the most recent answers (2015) date_format() is deprecated (I believe). Replace it with label_date() or scales::label_date(). It may not be loaded in your namespace but should come with ggplot so scales:: might be necessary.
Here's a copy and paste of the #timothyylim accepted answer with the change.
library(dplyr)
df_test1 = df_test %>% mutate(time = as.Date(time))
library(ggplot2)
library(scales)
p <- ggplot(df_test1, aes(time, reading))
p + geom_point(alpha = 1/4)+
scale_x_date(breaks="month", labels = scales::label_date("%b"))