summarise on Date column: min with na.rm=TRUE returns wrong value

[alexploner]

Unexpected behaviour when summarising a Date-variable using min (or max) under R 3.1.1, dplyr 0.3.0.2, Windows 7:

dates = as.Date(c("2014-01-01", "2013-01-01"))
dd = data.frame(Dates = dates)

Summarizing works fine:

require(dplyr)
summarise(dd, min(Dates))
##  min(Dates)
##1 2013-01-01

... except if I set add.rm=TRUE:

summarise(dd, min(Dates, na.rm=TRUE))
##  min(Dates, na.rm = TRUE)
##1               1970-01-01

Some implicit type conversion along the way?