Update data.table column changes data type

Question

I am testing a small scale scenario before rolling it out in a larger production environment and am experiencing a strange occurrence.

I have 2 data sets:

dtL <- data.table(URN=c(1,2,3,4,5), DonorType=c("Cash","RG","Emergency","Emergency","Cash"))
dtL[,c("EmergVal","EmergDate") := list(as.numeric(NA),as.Date(NA))]
setkey(dtL,URN)

dtR <- data.table(URN = c(1,1,1,2,3,3 ,3 ,4,4, 4,4,5),
                  class=c(5,5,5,1,5,40,40,5,40,5,40,5),
                  xx=   c(25,50,25,10,100,20,25,20,40,35,20,25),
                  xdate=as.Date(c("2013-01-01","2013-06-05","2014-05-27","2014-10-14",
                          "2014-06-09","2014-04-07","2014-10-16",
                          "2014-07-16","2014-10-21","2014-10-22","2014-09-18","2013-12-19")))
setkey(dtR,URN)

I am wanting to update dtL where the DonorType is equal to "Emergency", but only for a subset of records from dtR. I have seen Update subset of data.table based on join and thus have used that as a foundation for my solution.

dtL[dtR[class==40,list(maxxx=max(xx)),by=URN],
    EmergVal := ifelse(DonorType=="Emergency",i.maxxx,as.numeric(NA))]
dtL[dtR[class==40,list(maxdate=max(xdate)),by=URN],
    EmergDate := ifelse(DonorType=="Emergency",as.Date(i.maxdate),as.Date(NA)),nomatch=0]

I don't get any errors, however when I look at the data now in dtL it has changed the datatype for EmergDate to num rather than what it originally was (i.e. Date).

So three questions

1. Why has it changed the data type (especially when it is a Date when first created in dtL, and I tell it to put it as a date in my ifelse statement?
2. How do I get it to keep the date type when I assign it? or will I have to do some post assignment conversion/castint?
3. Is there a clean way I could do my assignment of EmergVal and EmergDate in a single statement given that I don't have a field DonorType in dtR and I don't want to add it in (so can't use a multiple key for the join)?