R - Date & Time

• Date
• Represented by the Date class, stored internally as number of days since 1970-01-01

 > x <- as.Date("2014-09-09")  # create date
 > x  
 [1] "2014-09-09"  
 > unclass(x)  
 [1] 16322  # number of days since 1970-01-01

• Time
• Representd by the POSIXct or POSIXlt class
• POSIXct: large integer

 > y <- Sys.time()  
 > y <- as.POSIXct(y)  
 > names(unclass(y))  
 NULL  
 > unclass(y)  
 [1] 1411258804  

• POSIXlt: with many useful information

 > y <- as.POSIXlt(y)  
 > names(unclass(y))  
  [1] "sec"  "min"  "hour"  "mday"  "mon"  "year"   
  [7] "wday"  "yday"  "isdst" "zone"  "gmtoff"  

• Create from string

 > time <- c("September 21, 2014 1:27")  
 > x <- strptime(time, "%B %d, %Y %H:%M")  
 > x  
 [1] "2014-09-21 01:27:00 BST"  
 > class(x)  
 [1] "POSIXlt" "POSIXt"   

• Check time differences

 > y <- Sys.time()  
 > y  
 [1] "2014-09-21 01:34:42 BST"  
 > x - y  
 Time difference of -7.709296 mins