Date

Base R
df[,1] = as.Date(df[,1],format="%m/%d/%y") class(df$Datum) "%Y-%m-%d" = 2009-05-31 "%Y/%m/%d" = 2009/05/31 "%y/%m/%d" = 09/05/31 "%d%b%Y"  = 31jan1960

Lubridate Package
library(lubridate) DF$Date = lubridate::ymd(DF$Date)

Create sequence of time
date_list = seq(ymd('2005-04-16'),ymd('2016-10-15'),by='day') date_list = date_list[which(month(date_list) %in% c(4:10))] # May to October datelist = seq(ymd_hms('1980-01-01 00:00:00'),ymd_hms('1980-07-01 00:00:00'), by = '60 mins')

Modify Data based on Time
idx = which(testframe$Date >= "2015-01-01" & testframe$Date <= "2015-01-27",) testframe[idx,][,2:5] = testframe[idx,][,2:5] * 2