Convert an FGDB sourced datetime column to a POSIXct type in R

handle_datetime.R

library('arcgisbinding')
arc.check_product()

# a FGDB source with a column 'datetime' containing our datetime field
d <- arc.open('a/fgdb/source')
df <- arc.select(d, 'datetime')

# convert from 1899-12-30 00:00 to UNIX Epoch, in seconds
as.unix.epoch <- (df$datetime[1]-25569)*86400

# convert to POSIXct object, albeit with invalid TZ
ct <- as.POSIXct(as.unix.epoch, origin="1970-1-1", tz="GMT")

# fix the TZ
fmt <- paste("%Y-%m-%d %H:%M:%S%z ", Sys.timezone())
ct.with.tz <- as.POSIXct(format(ct, fmt), usetz=TRUE)