ggplot2和x轴上的日期

all. I'm having problems plotting a time series using ggplot2. I am relatively new to R and cannot for the life of me figure out why R cannot execute my code. I get the following error: "Invalid input: date_trans works with objects of class Date only". What exactly is this error trying to tell me? I've Googled this error before, and there doesn't seem to be an explanation of what this error means--just code to eliminate the error. all. I'm having problems plotting a time series