How to Make Time Difference in Same Units When Subtracting Posixct

How to make time difference in same units when subtracting POSIXct

You can use difftime for that propose which allows you to specify the measurement units, for example

difftime(t1, t2, units = "secs")

Another way (as mentioned by @nicola and is present in the same documentation) is to take advantage of the fact that - has a -.POSIXt method and override the measurement units after the subtraction operation using units<- replacement method

res <- t1 - t2
units(res) <- "secs"

Get time difference always in minutes

Instead of just subtracting one time from another, use difftime so you can specify the unit as minutes:

start <- Sys.time()
end <- Sys.time()


difftime(end, start, units = 'mins')
#Time difference of 0.1102342 mins

Time difference from two POSIXct objects is not calculated correctly

Have you ever heard of daylight saving time?

x
#[1] "2013-03-31 01:39:42 CET"
y
#[1] "2013-03-31 03:11:24 CEST"

Look at the time zones.

Compare with this:

x = as.POSIXct('2013-03-31 01:39:42', tz="GMT")
y = as.POSIXct('2013-03-31 03:11:24', tz="GMT")
y-x
#Time difference of 1.528333 hours

How to get time differences with output having multiple units

Since you would like days, hours, minutes, seconds, we can get this result with the lubridate package:

a<-c("2018-03-20 11:52:25 AST", "2018-03-20 12:51:25 AST", "2018-03-20 14:19:04 AST",
 "2018-03-21 14:12:12 AST", "2018-03-21 12:09:22 AST", "2018-03-21 15:28:01 AST")

b<-c("2018-04-09 18:39:38 AST", "2018-06-23 19:13:14 AST", "2018-03-20 23:23:03 AST",
 "2018-05-10 21:29:28 AST", "2018-03-22 03:17:23 AST", "2018-05-12 00:19:39 AST")

a = as.POSIXct(a)
b = as.POSIXct(b)

library(lubridate)
timespan = interval(ymd_hms(ab[,1]), ymd_hms(ab[,2]))
> as.period(timespan)
[1] "20d 6H 47M 13S"    "3m 3d 6H 21M 49S"  "9H 3M 59S"         "1m 19d 7H 17M 16S"
[5] "15H 8M 1S"         "1m 20d 8H 51M 38S"

If desired, we can convert months to days by specifying the formatting as follows:

> as.period(timespan, unit = "day")
[1] "20d 6H 47M 13S" "95d 6H 21M 49S" "9H 3M 59S"      "50d 7H 17M 16S"
[5] "15H 8M 1S"      "51d 8H 51M 38S"

How to change the units of difftime after computing the values, and not using units=xxx when performing the calculations

What is returned by difftime is an object of class "difftime".

Other function have methods for difftime. For example, to convert to the number of hours, as a numeric:

as.numeric(difftime("2015-12-07", "2015-12-05"), units="hours")

[1] 48

Or, to get weeks:

as.numeric(difftime("2015-12-07", "2015-12-05"), units="weeks")

[1] 0.2857143

Also, you might find it useful to remember that POSIXct objects are actually numbers! They represent the number of seconds elapsed since ‘1970-01-01 00:00.00 UTC’, (assumes Gregorian Calendar).

As a result, it can often be convenient to think of a unit of time of fixed duration (i.e., not something like "month", which isn't constant) and perform calculations using it. E.g., once your time difference is in seconds, it's to convert to other constant (technically there is a very fine scale variation in some of these, but most applications don't require attention to these details) time units like minutes, hours, days, or weeks.

R Find time difference in seconds for YYYY-MM-DD HH:MM:SS.MMM format

Easy peasy:

R> now <- Sys.time()
R> then <- Sys.time()
R> then - now
Time difference of 5.357 secs
R> class(then - now)
[1] "difftime"
R> as.numeric(then - now)
[1] 5.357
R> 

And for your data:

R> df
                    time2                   time3
1 2011-09-01 23:44:52.533 2011-09-01 23:43:59.752
2  2011-09-05 12:25:37.42 2011-09-05 12:25:01.187
3  2011-08-24 12:56:58.91 2011-08-24 12:55:13.012
4 2011-10-25 07:18:14.722 2011-10-25 07:16:51.759
5 2011-10-25 07:19:51.697 2011-10-25 07:16:51.759
R> df$time2 <- strptime(df$time2, "%Y-%m-%d %H:%M:%OS")
R> df$time3 <- strptime(df$time3, "%Y-%m-%d %H:%M:%OS")
R> df
                    time2                   time3
1 2011-09-01 23:44:52.533 2011-09-01 23:43:59.752
2 2011-09-05 12:25:37.420 2011-09-05 12:25:01.187
3 2011-08-24 12:56:58.910 2011-08-24 12:55:13.012
4 2011-10-25 07:18:14.722 2011-10-25 07:16:51.759
5 2011-10-25 07:19:51.697 2011-10-25 07:16:51.759
R> df$time2 - df$time3
Time differences in secs
[1]  52.781  36.233 105.898  82.963 179.938
attr(,"tzone")
[1] ""
R> 

and added back as a numeric to the data frame:

R> df$dt <- as.numeric(df$time2 - df$time3)
R> df
                    time2                   time3      dt
1 2011-09-01 23:44:52.533 2011-09-01 23:43:59.752  52.781
2 2011-09-05 12:25:37.420 2011-09-05 12:25:01.187  36.233
3 2011-08-24 12:56:58.910 2011-08-24 12:55:13.012 105.898
4 2011-10-25 07:18:14.722 2011-10-25 07:16:51.759  82.963
5 2011-10-25 07:19:51.697 2011-10-25 07:16:51.759 179.938
R> 

How to add/subtract time from a POSIXlt time while keeping its class in R?

Short answer: No

Long answer:

POSIXct and POSIXlt objects are two specific types of the more general POSIXt class (not in a strictly object oriented inheritance sense, but in a quasi-object oriented implementation sense). Code freely switches between these. When you add to a POSIXlt object, the actual function used is +.POSIXt, not one specifically for POSIXlt. Inside this function, the argument is converted into a POSIXct and then dealt with (added to).

Additionally, POSIXct is the number of seconds from a specific date and time. POSIXlt is a list of date parts (seconds, minutes, hours, day of month, month, year, day of week, day of year, DST info) so adding to that directly doesn't make any sense. Converting it to a number of seconds (POSIXct) and adding to that does make sense.

Time difference between rows in R dplyr, different units

ts <- ts %>% group_by(MDN) %>% arrange(Cl_Date) %>%
  mutate(time_diff_2 = as.numeric(Cl_Date-lag(Cl_Date), units = 'mins'))

Convert the time difference to a numeric value. You can use units argument to make the return values consistent.

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