Specifying colClasses in the read.csv

You can specify the colClasse for only one columns.

So in your example you should use:

data <- read.csv('test.csv', colClasses=c("time"="character"))

The colClasses vector must have length equal to the number of imported columns. Supposing the rest of your dataset columns are 5:

colClasses=c("character",rep("numeric",5))