grouped diff returns incorrect results for POSIXct objects

[MichaelChirico]

library(data.table)
DT = data.table(
  a = c(1, 1, 2, 2),
  b = structure(
    c(0, 10, 0, 100000000), 
    class = c('POSIXct', 'POSIXt')
  )
)
DT[ , diff(b), keyby = a]
#    a            V1
# 1: 1   10.000 secs
# 2: 2 1157.407 secs

The problem is that diff.POSIXt silently invokes -.POSIXt, which chooses units automatically given its inputs.

These units are ignore on group wrapup:

DT[ , {
  x = diff(b)
  dput(x)
  x
}, keyby = a]
# structure(10, units = "secs", class = "difftime")
# structure(1157.40740740741, units = "days", class = "difftime")
#    a            V1
# 1: 1   10.000 secs
# 2: 2 1157.407 secs

I suppose this is closely related with #995 and is almost identical to #761

[MichaelChirico]

Hmm actually duplicate of #761.