[R] Non date value

[Ashta]

Hi all,
I am reading  a field data that contains several variables. The sample
of the data with the first two variables is shown below.  I wanted to
know the minimum  and maximum recording date   However, I have some
problem.


Name      Rdate     V1 to  V20
Alex1    01/03/2015
Alex2    01/03/2014
Alex3    31/12/2012
Alex4    15/01/2011
Alex150  22/01/2010
Alex151  15/02/2011



DF1=DF1[!is.na(DF1$Rdate),]
range(DF1$Rdate, na.rm=TRUE)

Warning message:
In is.na(DF1$Rdate) :
  is.na() applied to non-(list or vector) of type 'NULL'
Error in DF1$Rdate : $ operator is invalid for atomic vectors
Execution halted

I am expecting the Rdate field should contain  recording dates. I  am
suspecting there might be a non date  value in that columns. How do I
remove that row if it is not a date format?


Thank you.