modify hour of a subset using lubridate

modify_hour.r

library(lubridate)
## A synthetic data set. 
doc <- data.frame(date=ymd_hm("2014-01-01 14:00") + (1:360)* 24*3600)
# Find all the dates before a certain time period. 
index <- doc$date < ymd("2014-06-01")
## use the hour function to extract the hour of each day e.g
hour(doc$date)
## Now using the index from above you can subtract an hour from each of the times
# using the index vector and reassiging the modified hour back to the time stamp. 
hour(doc$date[index]) <- hour(doc$date[index]) - 1