Date and DateTime in Spotfire 7.x used with sdcGetData function parameters

Question

does anyone know how to use Date / DateTime Spotfire types as parameters for sdcGetData in R?

I'm trying to access data in our Spotfire library using the SpotfireData library availble in TERR. I follow the instructions in the documentation here. I have an information-link I'm trying retrieve data from which has two prompts defined (Product, StartDate). When I run sdcGetInfo I get the following.

Spotfire data connection info
  server: http://OURSPOTFIRESERVER/spotfire
  path: /INFOLINKPATH
  1 view

Spotfire data connection view: "INFOLINKPATH"
  Parameters:
       Name DataType Required AllowMultipleValues IsRangeValues
1 StartDate     Date     TRUE                TRUE          TRUE
2   Product   String     TRUE                TRUE         FALSE

I then try to query data with

df <- sdcGetData(conn, "/INFOLINKPATH", Product = "ABCD", StartDate = c(as.POSIXct("2018-01-01"), as.POSIXct("2018-06-01")))

Which does not work and returns

Error in sdcHttp(conn, url, body = NULL, headers = headers, sendCooki : http err
or: status=500

Now I tried several things with the data including as.Date, using strings, as.POSIXlt, adding timezone information. I've also tried not using RANGE type prompt and instead single value but still get 500 error. If I remove the date as a prompt it works and returns data. Spotfire does not like my dates used as parameters but I cannot figure out why.