I have been trying to process a data.frame (tr) on a grouping factor (Rmbr) using diff
to find differences between srv_dt (a date) by Rmbr and keep getting the following error message

> tmp <- by(tr,tr$Rmbr, diff, tr$srv_dt, lag=1, difference=1)
Error in r[i1] - r[-length(r):-(length(r) - lag + 1L)] :
  non-numeric argument to binary operator
# the only two arguments I supply are both numeric
> mode(tr$srv_dt)
[1] "numeric"
> mode(tr$Rmbr)
[1] "numeric"


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