Set units of difference between datetime objects

Question

The diff command returns the differences between dates in a vector of dates in the R date format. I'd like to control the units that are returned, but it seems like they are automatically determined, with no way to control it w/ an argument. Here's an example:

 > t = Sys.time() 
 > diff(c(t, t + 1))
 Time difference of 1 secs

And yet:

> diff(c(t, t+10000))
Time difference of 1.157407 days

The "time delta" object has a units attribute, but it seems silly to write a bunch of conditionals to coerce everything into days, seconds etc.

Answer 1

I'm not sure what you mean by "a bunch of conditionals," just change the units manually.

> t = Sys.time()
> a <- diff(c(t,t+1))
> b <- diff(c(t, t+10000))
> units(a) <- "mins"
> units(b) <- "mins"
> a
Time difference of 0.01666667 mins
> b
Time difference of 166.6667 mins

See ?difftime. If you only need to use diff to get the difference between two times (rather than a longer vector), then, as Dirk suggests, use the difftime function with the units parameter.

Answer 2

A POSIXct type (which you created by calling Sys.time()) always use fractional seconds since the epoch.

The difftime() functions merely formats this differently for your reading pleasure. If you actually specify the format, you get what you specified:

R> difftime(t+ 10000,t,unit="secs")
Time difference of 10000 secs
R> difftime(t+ 10000,t,unit="days")
Time difference of 0.115741 days
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