R1,9,0beta: strange summary of Date object

Hola!

Any explanation of the following:

summary(as.Date(rep(NA,20)))

Min.      1st Qu.       Median         Mean      3rd Qu.      
   Max. 
          NA           NA           NA           NA           NA      
     NA 
        NA's 
"1970-01-21" 

Where does this 1970 date come from?

Kjetil Halvorsen