Subtract hours from date [duplicate]

Question

I am using the Openair package of R. First step is importing CSV. It all works fine except I need to shift the time/date in that CSV by 6 hours back. In other words, for example change midnight of one day to 6 PM of the previous day etc. This is so that the daily averages are calculated 6AM-6AM instead of 0-0.

So far I have:

library(openair)
library(tidyverse)

x <- import(file = "data.csv", file.type = "csv", sep = ";", header.at = 1, data.at = 2, date = "date", date.format = "%d.%m.%Y", time = "hour", time.format = "%H",quote = "\"")

The CSV has a column "date", which stores date as d.m.Y and a field hour, which is a numeric value of the hour.

Later I select a particular year, but this needs to be done already after the subtraction

dataSet <- selectByDate(x, year = 2018)

Answer 1

library(lubridate)

df <- data.frame(datetime = ymd_hms(c("2018-01-01 05:00:00","2018-01-01 06:00:00", 
"2018-01-01 07:00:00", "2018-01-01 08:00:00")))

print(df)
#datetime
# 2018-01-01 05:00:00
# 2018-01-01 06:00:00
# 2018-01-01 07:00:00
# 2018-01-01 08:00:00


df$datetime_adjusted <- df$datetime - hours(6)

print(df)
#datetime   datetime_adjusted
# 2018-01-01 05:00:00 2017-12-31 23:00:00
# 2018-01-01 06:00:00 2018-01-01 00:00:00
# 2018-01-01 07:00:00 2018-01-01 01:00:00
# 2018-01-01 08:00:00 2018-01-01 02:00:00