How to make time difference in same units when subtracting POSIXct

Question

I want to subtract to POSIXct. I can do this but depending on the first row (i guess?) the difference will be in seconds or minutes. Below you can see the first diff is in seconds and the second diff is in minutes because I changed the time difference in the first row:

#diff in seconds because 1st row time diff is small?
t1<- as.POSIXct(c("2015-02-02 20:18:03 00:00:00", "2015-02-02 20:17:02 00:00:00"),"GMT")
t2<- as.POSIXct(c("2015-02-02 20:18:02 00:00:00","2015-02-02 20:18:02 00:00:00"),"GMT")
d<-data.frame(t1= t1, t2= t2)
d$t1-d$t2


#diff in seconds because 1st row time diff is larger?
t1<- as.POSIXct(c("2015-02-02 20:13:03 00:00:00", "2015-02-02 20:17:02 00:00:00"),"GMT")
t2<- as.POSIXct(c("2015-02-02 20:18:02 00:00:00","2015-02-02 20:18:02 00:00:00"),"GMT")
d<-data.frame(t1= t1, t2= t2)
d$t1-d$t2

results:

> #diff in seconds because 1st row time diff is small?
> t1<- as.POSIXct(c("2015-02-02 20:18:03 00:00:00", "2015-02-02 20:17:02 00:00:00"),"GMT")
> t2<- as.POSIXct(c("2015-02-02 20:18:02 00:00:00","2015-02-02 20:18:02 00:00:00"),"GMT")
> d<-data.frame(t1= t1, t2= t2)
> d$t1-d$t2
Time differences in secs
[1]   1 -60
> 
> 
> #diff in seconds because 1st row time diff is larger?
> t1<- as.POSIXct(c("2015-02-02 20:13:03 00:00:00", "2015-02-02 20:17:02 00:00:00"),"GMT")
> t2<- as.POSIXct(c("2015-02-02 20:18:02 00:00:00","2015-02-02 20:18:02 00:00:00"),"GMT")
> d<-data.frame(t1= t1, t2= t2)
> d$t1-d$t2
Time differences in mins
[1] -4.983333 -1.000000

I would like the difference to ALWAYS be in seconds no matter what the first row difference is. Is there a way to make this happen?

Thank you.

Solution

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"

Comments

[David Arenburg]

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

[nicola]

Or, if you want to use - instead of difftime, change the unit through units<-. For instance: x<-d$t1-d$t2; units(x)<-"secs"

[David Arenburg]

@user20650, Ok added. Hope nicola doesn't mind I blended his comment too.

[HappyCoding]

units(res) <- "secs" seems not correct. it will also read 6 days as 6 seconds

[David Arenburg]

@HappyCoding Not sure what you mean. res <- as.POSIXct("2015-05-02 20:18:02 00:00:00") - as.POSIXct("2015-02-02 20:18:02 00:00:00") ; units(res) <- "secs" seem to work fine.

[HappyCoding]

yes, above can work. My fault. seems it can only work with element instead of vector or data frame. my code example > res <- as.POSIXct("2015-05-02 20:18:02 00:00:00") - as.POSIXct("2015-02-02 20:18:02 00:00:00") > res <- rbind(res,res) > as.numeric(res,units="secs")

[David Arenburg]

@HappyCoding That's because rbind caused it to lose its difftime class and units only works with that class (as documented under ?units). This works on vectors as long they have the proper class. Try res <- as.POSIXct(c("2015-05-02 20:18:02 00:00:00", "2015-05-02 20:18:02 00:00:00")) - as.POSIXct(c("2015-02-02 20:18:02 00:00:00", "2015-02-02 20:18:02 00:00:00")) ; units(res) <- "secs" for instance.

[HappyCoding]

thanks @David. Yes, you're right. I solved the problem with lappy to each instance.