TERR converts Spotfire dates to milliseconds from 1970.  My understanding is that it doesn't have a timezone associated with the datetimes, so it is considered 'UTC'.

Therefore, inside TERR, you need to manually convert the imported Spotfire datetimes to something that TERR can operate on.

This is an in place conversion:

data$MinDate = as.POSIXct(data$MinDate, origin="1970-01-01", tz="UTC")